superio/nuvoton: factor out generic romstage components

The romstage of Nuvoton Super I/O chips (but not Nuvoton BMC chips)
is identical, so the early_serial.c file can be moved under
nuvoton/common.
The Nuvoton BMC chip WPCM450 is however left untouched.

Change-Id: I4663176c1003b24a49a9fe5f9ebd27a1963b5565
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: http://review.coreboot.org/5909
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Dave Frodin <dave.frodin@se-eng.com>