lib: utils: Move PLIC DT fix up codes to fdt_helper.c

Now that we have a dedicated fdt_helper.c file for DT releated
helper routines, move plic_fdt_fixup() codes from plic.c to
fdt_helper.c and rename it to fdt_plic_fixup() at the same time,
to keep name consistency in the same file.

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