lib: utils: Improve fdt_cpu_fixup() implementation

Currently, the fdt_cpu_fixup() implementation assumes:
1. We have one CPU DT for each HART under /cpus DT node
2. The CPU DT nodes are named sequentially (i.e cpu@0,
   cpu@1, ...) which is not true for discontinuous and
   sparse HART ids (i.e. cpu@0, cpu@4, cpu@5). Generally,
   CPU DT node are named based on HART id and not HART
   index

If any of the above assumptions are violated then the
fdt_cpu_fixup() will not work.

This improves fdt_cpu_fixup() implementation and makes
it independent of above assumptions.

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