mb/google/zork: Disable UART 1, 2 and 3

We don't use these on zork, so lets save the power.

BUG=b:153001807
TEST=Boot OS and make sure UART 1, 2 and 3 are not probed and remain
powered off.

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I2fadeba779b66ec2fb13951b9487118ef0737a94
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42328
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/mainboard/google/zork/variants/baseboard/devicetree.cb

@@ -187,4 +187,8 @@ chip soc/amd/picasso
 		end
 	end
 
+	device mmio 0xfedca000 off end # UART1
+	device mmio 0xfedce000 off end # UART2
+	device mmio 0xfedcf000 off end # UART3
+
 end	# chip soc/amd/picasso