serial: 8250: of: Fix the driver and actually compile the 8250_of

The 8250_of never compiled since in the Kconfig we have SERIAL_OF_PLATFORM
but in the makefile we expect to have SERIAL_8250_OF...

When the 8250_of.c is actually compiled we will have two errors:
missing linux/nwpserial.h and 8250/8250.h.
Fix those as well at the same time when enable the compilation of the
driver.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Fixes: afd7f88f ("serial: 8250: move of_serial code to 8250 directory")
Reported-by: Guenter Roeck <linux@roeck-us.net>
CC: Arnd Bergmann <arnd@arndb.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>