fdt: Add fdt_getprop_u32_default helpers

Add helper functions to return find a node and return it's property
or a default value.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Andrew Fleming-AFLEMING <afleming@freescale.com>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>