mb/amd/onyx_poc/devicetree: enable UART0

UART0 is routed to a USB-serial converter chip on the Onyx board, so
also enable this UART in the devicetree, so that the OS will be able to
use this UART.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I7b2577d799fd82a0aa0c9b01324930237e204aa0
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79651
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Varshit Pandya <pandyavarshit@gmail.com>

src/mainboard/amd/onyx_poc/devicetree.cb

@@ -200,6 +200,7 @@ chip soc/amd/genoa_poc
 		end
 	end
 
+	device ref uart_0 on end
 	device ref uart_1 on end
 
 end