mb/system76/lemp9: skip FSP init of UART2

This UART is already initialized by coreboot for the console, it does
not need to be initialized by the FSP.

Tested on lemp9.

Signed-off-by: Jeremy Soller <jeremy@system76.com>
Change-Id: I7c299fd7cf6fe53d1f500a899a14e63e51ad6266
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43676
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

src/mainboard/system76/lemp9/devicetree.cb

@@ -43,6 +43,7 @@ chip soc/intel/cannonlake
 	# Serial I/O
 	register "SerialIoDevMode" = "{
 		[PchSerialIoIndexI2C0] = PchSerialIoPci, // Touchpad
+		[PchSerialIoIndexUART2] = PchSerialIoSkipInit, // LPSS UART
 	}"
 
 	# Misc