serial: 8250: Default SERIAL_OF_PLATFORM to SERIAL_8250

It is way too easy to miss enabling SERIAL_OF_PLATFORM which would
result in the inability for the kernel to have a valid console device,
which can be seen with:

Warning: unable to open an initial console.

and then:

Run /init as init process
Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100

Since SERIAL_OF_PLATFORM already depends on SERIAL_8250 && OF there
really is no drawback to defaulting this config to the value of
SERIAL_8250.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>