lib: utils: Improve fdt_cpu_fixup() implementation

The fdt_cpu_fixup() should work fine even if HARTs without MMU
are not marked invalid by platform support code.

In future, we plan to treat HARTs without MMU as valid in the
generic platform support so that we can hold these HARTs in
HSM STOPPED state.

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