mb/asus/p8z77-m: Properly configure early serial

Board was not producing serial output until well into ramstage.

To fix, select SUPERIO_NUVOTON_COMMON_COM_A Kconfig to tell
nuvoton_enable_serial() to route serial port A signals to the outside,
not GPIO8x.

TEST=Full native raminit debug log received over serial by minicom.

Change-Id: I376a79dd76ffa5f4d47e7c0cb53680e173e1ad78
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79222
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/asus/p8x7x-series/Kconfig

@@ -54,6 +54,7 @@ config BOARD_ASUS_P8Z77_M
 	select BOARD_ROMSIZE_KB_8192
 	select MEMORY_MAPPED_TPM
 	select SUPERIO_NUVOTON_NCT6779D
+	select SUPERIO_NUVOTON_COMMON_COM_A
 
 if BOARD_ASUS_P8X7X_SERIES