powerpc: Permit device tree control of U-Boot (CONFIG_OF_CONTROL)

Enable this in the Kconfig so that PowerPC boards can use device tree to
configure U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/Kconfig

@@ -40,6 +40,7 @@ config OPENRISC
 config PPC
 	bool "PowerPC architecture"
 	select HAVE_PRIVATE_LIBGCC
+	select SUPPORT_OF_CONTROL
 
 config SANDBOX
 	bool "Sandbox"