pylibfdt: Add support for fdt_get_alias()

Add this into the class to simplify use of this function.

Signed-off-by: Appana Durga Kedareswara rao <appana.durga.rao@xilinx.com>
Message-Id: <1562130487-27028-1-git-send-email-appana.durga.rao@xilinx.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>