soc/intel/alderlake: Drop unused PrmrrSize from devicetree

The `PrmrrSize` FSP-M UPD is set using `get_valid_prmrr_size()`. As the
devicetree option's value is not used anywhere, drop it.

Change-Id: Ib6fb77b03a4648adbd8b23c160cfba94d142a2d2
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52108
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/intel/adlrvp/devicetree_m.cb

@@ -35,8 +35,6 @@ chip soc/intel/alderlake
 	# EC memory map range is 0x900-0x9ff
 	register "gen3_dec" = "0x00fc0901"
 
-	register "PrmrrSize" = "0"
-
 	#Enable PCH PCIE RP 4 using CLK 5
 	register "pch_pcie_rp[PCH_RP(4)]" = "{
 		.clk_src = 5,