serial: 8250: move of_serial code to 8250 directory

As the of-serial driver is now 8250 specific, we can move the
file to a more appropriate place in teh 8250 subdirectory and
adapt the Kconfig help text and file name.

I'm leaving the CONFIG_SERIAL_OF_PLATFORM symbol unchanged
to avoid breaking user configuration files unnecessarily.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>