Raspberry Pi: add device tree overlay support

Raspberry Pi has numerous HAT and other HW expansion options such as screens and
cameras some of which need overlays from the beginning so it makes sense to
enable evice tree overlays.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
diff --git a/configs/rpi_defconfig b/configs/rpi_defconfig
index 1b954fe..4aa2d3e 100644
--- a/configs/rpi_defconfig
+++ b/configs/rpi_defconfig
@@ -25,3 +25,5 @@
 CONFIG_SYS_WHITE_ON_BLACK=y
 CONFIG_CONSOLE_SCROLL_LINES=10
 CONFIG_PHYS_TO_BUS=y
+CONFIG_OF_LIBFDT=y
+CONFIG_OF_LIBFDT_OVERLAY=y