mb/starlabs/starfighter/mtl: Add detection delay to PCH-attached SSD

Some SSDs connected to the PCH-attached PCIe root port/m.2 socket need
a small delay in order to be reliably detected. Add a 15ms delay (the
default is 0) to ensure this.

TEST=build/boot Starfighter MTL 125H/285H with Samsung 970 EVO plus,
WD SN720, and Intel Optane P1600x SSDs in outer SSD socket. Ensure
all drives detected and bootable after both cold and warm resets.

Change-Id: I16ec0a313fc7cccb2807593c07db04cdbb59c979
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90880
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>

src/mainboard/starlabs/starfighter/variants/mtl/devicetree.cb

@@ -241,6 +241,7 @@ chip soc/intel/meteorlake
 				.flags					= PCIE_RP_LTR | PCIE_RP_AER,
 				.pcie_rp_aspm				= ASPM_L0S_L1,
 				.PcieRpL1Substates			= L1_SS_L1_2,
+				.pcie_rp_detect_timeout_ms		= 15,
 			}"
 
 			smbios_slot_desc				"SlotTypeM2Socket3"