mb/amd/onyx_poc/devicetree: Enable UART1

UART1 is selected for debug uart in Kconfig, it also needs to enabled
in devicetree. With this serial output doesn't stop during the device
enable step.

TEST:Build onyx_poc board

Change-Id: I7c910301c6eca5d3057785607139ac03b344bc15
Signed-off-by: Varshit Pandya <pandyavarshit@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79643
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/mainboard/amd/onyx_poc/devicetree.cb

@@ -200,4 +200,6 @@ chip soc/amd/genoa_poc
 		end
 	end
 
+	device ref uart_1 on end
+
 end