mb/intel/mtlrvp: Enable RTD3 root port mutex for WWAN

This adds RTD3 RPMX mutex to the root port. It is shared between RTD3
and WWAN. The purpose of using this mutex is to prevent OSPM from
calling _ON and _OFF methods while WWAN kernel driver is calling _RST,
which accesses the GPIO pins.

BUG=NA
TEST=boot to OS and check the generated SSDT table for the root port.
The RPMX mutex should be generated under the root port.

Signed-off-by: Cliff Huang <cliff.huang@intel.com>
Change-Id: I5b53765453bac0fc96e9651ab347069c7c8bf058
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73384
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Bora Guvendik <bora.guvendik@intel.com>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>

src/mainboard/intel/mtlrvp/variants/baseboard/mtlrvp_p/devicetree.cb

@@ -297,6 +297,7 @@ chip soc/intel/meteorlake
 				register "srcclk_pin" = "1"
 				register "ext_pm_support" = "ACPI_PCIE_RP_EMIT_ALL"
 				register "skip_on_off_support" = "true"
+				register "use_rp_mutex" = "true"
 				device generic 0 alias rp7_rtd3 on end
 			end
 			chip drivers/wwan/fm