mb/google/brya: Add `ext_pm_support` for volmar eMMC SKU

This patch ensures google/volmar eMMC SKU has advanced PM support
enabled.

BUG=b:235915257
TEST=Able to boot to eMMC SKU to ChromeOS.

Signed-off-by: Subrata Banik <subratabanik@google.com>
Change-Id: I3e2883d894d2ca7f810f4b72af1c12037c8fdabc
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65244
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ren Kuo <ren.kuo@quanta.corp-partner.google.com>

src/mainboard/google/brya/variants/volmar/overridetree.cb

@@ -304,6 +304,7 @@ chip soc/intel/alderlake
 			chip soc/intel/common/block/pcie/rtd3
 				register "reset_gpio" = "ACPI_GPIO_OUTPUT_ACTIVE_LOW(GPP_B4)"
 				register "srcclk_pin" = "4"
+				register "ext_pm_support" = "ACPI_PCIE_RP_EMIT_ALL"
 				device generic 0 alias emmc_rtd3 on end
 			end
 			# Enable PCIe-to-eMMC bridge PCIE 3 using clk 4