Revert "serial: 8250: Default SERIAL_OF_PLATFORM to SERIAL_8250"

This reverts commit 6d11023c.

It causes issues with Guenter's test systems and since there seems to
not be any agreement about _why_ this is a problem, but reverting it
fixes things, let's revert until the root cause is found.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>