pc_piix/pc_sysfw: enable flash by default

Now, the pc-sysfw:rom_only property will default
to false which enables flash by default.

All pc types below pc-1.1 set rom_only to true.
This prevents flash from being enabled on these
pc machine types.

For pc-1.1 rom_only will use the default (false),
which will allow flash to be used for pc-1.1.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
