lib, fdt: Adding fdtdec_get_uint function

Adding fdtdec_get_uint function which is the
unsigned version for fdtdec_get_int

Signed-off-by: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Dinh Nguyen <dinh.linux@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Vikas Manocha <vikas.manocha@st.com>
Cc: Jagannadh Teki <jteki@openedev.com>
Cc: Pavel Machek <pavel@denx.de>
Cc: Heiko Schocher <hs@denx.de>