[LIBS] libfdt: Fix libfdt_get_property() API

It can happen that "property_value" pointer passed to
libfdt_get_property() API points to memory of length
less than actual length of property in FDT. In this
situation the libfdt_get_property() API will corrupt
the memory pointed by "property_value" pointer.

This patch fixes libfdt_get_property() API by introducing
"property_len" argument which denotes the length of
memory pointed by "property_value" pointer.

Signed-off-by: Anup Patel <anup@brainfault.org>