BUG: DatetimeIndex.to_period().to_timestamp() forgets freq value
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
>>> pd.date_range(start='1/1/2018 00:00:00', end='2/1/2018 00:01:00', freq="MS")
DatetimeIndex(['2018-01-01', '2018-02-01'], dtype='datetime64[ns]', freq='MS')

>>> pd.date_range(start='1/1/2018 00:00:00', end='2/1/2018 00:01:00', freq="MS").to_period().to_timestamp()
DatetimeIndex(['2018-01-01', '2018-02-01'], dtype='datetime64[ns]', freq=None)

```

#### Problem description

Why is `freq=None` here in the end? Is it possible to make conversion between `DatetimeIndex` and `PeriodIndex` more reliable in any way? How can I know which `freq` values are not able to be converted from `PeriodIndex` into `DatetimeIndex`?

####  Expected Output:
I would expect to have the same `freq` value after doing `.to_period().to_timestamp()`. 

```python
>>> pd.date_range(start='1/1/2018 00:00:00', end='2/1/2018 00:01:00', freq="MS").to_period().to_timestamp()
DatetimeIndex(['2018-01-01', '2018-02-01'], dtype='datetime64[ns]', freq="MS")
```

#### Output of ``pd.show_versions()``
``````
<details>

INSTALLED VERSIONS
------------------
commit           : 3e89b4c4b1580aa890023fc550774e63d499da25
python           : 3.7.9.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : AMD64 Family 21 Model 112 Stepping 0, AuthenticAMD
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.2.0
numpy            : 1.19.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.3.1
setuptools       : 51.0.0.post20201207
Cython           : 0.29.17
pytest           : 6.1.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 0.8.4
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.4
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.50.1
</details>
