mb/facebook/watson/v2: Reserve memory region for CPLD access

For watson_v2 variant, add lpc_lgmr register in device tree
configuration to access CPLD regsiters.

TESTED=booted watson_v2 server into target OS, confirm
CPLD register access:
devmem2 0xB0000100
Value at address 0xb0000100: 0x00020003

Signed-off-by: Jonathan Zhang <jonzhang@fb.com>
Signed-off-by: Ravi Rama <rrama@arista.com>
Change-Id: I43ee89b8609c64bccf5a21171d8ff192e6aca0ef
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51718
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/facebook/watson/variants/watson_v2/devicetree.cb

@@ -1,4 +1,5 @@
 chip soc/intel/fsp_broadwell_de
+	register "lpc_lgmr" = "0xb0000001" # CPLD
 	device cpu_cluster 0 on
 		device lapic 0 on end
 	end