lib: utils: Add fdt_parse_hart_id() function

Parsing HART id from a CPU DT node is a common requirement for
RISC-V systems. The newly added fdt_parse_hart_id() also helps
reduce duplicate code between fdt_cpu_fixup() function and
fdt_parse_hart_count() function.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>