lib: utils: Add a fdt_reserved_memory_fixup() helper

Add a helper routine to insert a child node of the reserved memory
node in the device tree that describes the protected memory region
done by OpenSBI via PMP.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>