Timestamp incorrectly handles datetime64 with exotic units
#### Code Sample, a copy-pastable example if possible

```python
>>> pd.Timestamp(np.datetime64('2019-01-01', '6h'))
Timestamp('1978-03-02 20:00:00')
```
#### Problem description

The `pd.Timestamp` constructor gives the wrong result when converting a `np.datetime64` that uses multiples of a standard unit. In the example above, I create a datetime64 with units of `6h` but the conversion appears to assume units of `s`.

This happens with units of `6h` but not with units of `h`:

```python
>>> pd.Timestamp(np.datetime64('2019-01-01', 'h'))
Timestamp('2019-01-01 00:00:00')
```

#### Expected Output

Pandas should either perform a correct conversion or raise a value error:

```python
>>> pd.Timestamp(np.datetime64('2019-01-01', '6h'))
Timestamp('2019-01-01 00:00:00')
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-134-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.1
pytest: None
pip: 19.0.3
setuptools: 40.8.0
Cython: None
numpy: 1.16.2
scipy: 1.2.1
pyarrow: None
xarray: 0.11.3
IPython: None
sphinx: 1.8.4
patsy: None
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: None
feather: None
matplotlib: 3.0.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: 4.3.2
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
