mb/google/eve: Enable HotPlug on PCIe root port for WiFi

Enable HotPlug for the PCIe root port that the WiFi device
is on so the OS can re-train the link without needing a reboot
if it goes down unexpectedly at runtime.

BUG=b:72417777
TEST=enable HotPlug on Eve Root Port 0 (WiFi) and check in
linux that it is identified as a HotPlug capable root port.

Change-Id: Id2b7fc92c8c9128f0e28102eb5991bda7fbf6799
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://review.coreboot.org/23512
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/eve/devicetree.cb

@@ -146,6 +146,7 @@ chip soc/intel/skylake
 	register "PcieRpClkReqNumber[0]" = "1"
 	register "PcieRpAdvancedErrorReporting[0]" = "1"
 	register "PcieRpLtrEnable[0]" = "1"
+	register "PcieRpHotPlug[0]" = "1"
 	#RP 1 uses CLK SRC 1
 	register "PcieRpClkSrcNumber[0]" = "1"