libfdt: Add support for appending the values to a existing property

Some properties may contain multiple values, these values may need
to be added to the property respectively. this patch provides this
functionality. The main purpose of fdt_append_prop() is to append
the values to a existing property, or create a new property if it
dose not exist.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>