LoongArch: Always enumerate MADT and setup logical-physical CPU mapping

Some drivers want to use cpu_logical_map(), early_cpu_to_node() and some
other CPU mapping APIs, even if we use "nr_cpus=1" to hard limit the CPU
number. This is strongly required for the multi-bridges machines.

Currently, we stop parsing the MADT if the nr_cpus limit is reached, but
to achieve the above goal we should always enumerate the MADT table and
setup logical-physical CPU mapping whether there is a nr_cpus limit.

Rework the MADT enumeration:

1. Define a flag "cpu_enumerated" to distinguish the first enumeration
(cpu_enumerated=0) and the physical hotplug case (cpu_enumerated=1)
for set_processor_mask().

2. If cpu_enumerated=0, stop parsing only when NR_CPUS limit is reached,
so we can setup logical-physical CPU mapping; if cpu_enumerated=1,
stop parsing when nr_cpu_ids limit is reached, so we can avoid some
runtime bugs. Once logical-physical CPU mapping is setup, we will let
cpu_enumerated=1.

3. Use find_first_zero_bit() instead of cpumask_next_zero() to find the
next zero bit (free logical CPU id) in the cpu_present_mask, because
cpumask_next_zero() will stop at nr_cpu_ids.

4. Only touch cpu_possible_mask if cpu_enumerated=0, this is in order to
avoid some potential crashes, because cpu_possible_mask is marked as
__ro_after_init.

5. In prefill_possible_map(), clear cpu_present_mask bits greater than
nr_cpu_ids, in order to avoid a CPU be "present" but not "possible".

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>