Update libfdt to v1.6.1

Update libfdt to v1.6.1

Update libfdt to v1.6.1 of libfdt taken from git://github.com/dgibson/dtc.
This update is done to support device tree overlays.

A few minor changes are done to make it compatible with Xen:
    fdt_overlay.c: overlay_fixup_phandle()

        Replace strtoul() with simple_strtoul() as strtoul() is not available in
        Xen lib and included lib.h.

        Change char *endptr to const char *endptr. This change is required for
        using simple_strtoul().

    libfdt_env.h:
        Remaining Xen changes to libfdt_env.h carried over from existing
        libfdt (v1.4.0)

Signed-off-by: Vikram Garhwal <fnu.vikram@xilinx.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Tested-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>