libfdt: Add helpers for 64-bit integer properties

In device trees in the world, properties consisting of a single 64-bit
integer are not as common as those consisting of a single 32-bit, cell
sized integer, but they're common enough that they're worth including
convenience functions for.

This patch adds helper wrappers of fdt_setprop_inplace(), fdt_setprop() and
fdt_appendprop() for handling 64-bit integer quantities in properties.  For
better consistency with the names of these new *_u64() functions we also
add *_u32() functions as alternative names for the existing *_cell()
functions handling 32-bit integers.

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