mb/asus/p8z77-m: Enable Port 80 UART

Copied this bit from asus/p8z77-m_pro, without it a GRUB2 payload will
get stuck in an endless loop showing

Unknown key 0xff detected

whenever there is an USB device (such as a keyboard) connected.
In this mode GRUB2 is so busy showing this message repeatedly that no
other keypress ever gets handled, and thus no other remedy is possible
than a reset via mb pins and unplugging the USB device.

Signed-off-by: Fabian Groffen <grobian@gentoo.org>
Change-Id: Iebd433e2762a69241257e1b4f859319536a8d8f5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75138
Reviewed-by: Keith Hui <buurin@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asus/p8x7x-series/variants/p8z77-m/overridetree.cb

@@ -80,6 +80,7 @@ chip northbridge/intel/sandybridge
 					end
 					device pnp 2e.609 off end	# GPIO 6
 					device pnp 2e.709 off end	# GPIO 7
+					device pnp 2e.14 on end		# Port 80 UART
 				end
 				chip drivers/pc80/tpm
 					device pnp 4e.0 on end	# TPM