rpi: Enable UART on RPi 4 series devices

U-Boot cannot boot systems on RPi 4 series until enable UART. But,
U-Boot cannot boot systems on RPi 5 series until disable UART. So, make
it only enable UART on RPi 4 series devices.

https://phabricator.endlessm.com/T30780#883184
https://app.asana.com/1/1203676861188277/project/1211061530470838/task/1211943756834438

Author: starnight

data/rpi-common/config.txt

@@ -39,6 +39,10 @@ disable_overscan=1
 # Run as fast as firmware / board allows
 arm_boost=1
 
+[pi4]
+# U-Boot cannot boot systems on RPi 4 series until enable UART
+enable_uart=1
+
 [cm4]
 # Enable host mode on the 2711 built-in XHCI USB controller.
 # This line should be removed if the legacy DWC2 controller is required