fdt: Change fdt_pack_reg() to static and fix types

This function is only called within this file so make it static. Also
fix its argument types to be consistent with its caller.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>