mb/amd/mandolin: enable SoC UARTs 0 and 1 and disable 2 and 3

There are only headers for the SoC's UART 0 and 1 on the board.

BUG=b:165020060
TEST=Linux only detects UART 0 and 1.

Change-Id: I45929f65a5f844ae5cef792b11176f487c80766f
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44530
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/amd/mandolin/variants/mandolin/devicetree.cb

@@ -155,4 +155,10 @@ chip soc/amd/picasso
 		device pci 18.6 on  end
 		device pci 18.7 on  end
 	end # domain
+
+	device mmio 0xfedc9000 on  end # UART0
+	device mmio 0xfedca000 on  end # UART1
+	device mmio 0xfedce000 off end # UART2
+	device mmio 0xfedcf000 off end # UART3
+
 end	# chip soc/amd/picasso