MIPS: Loongson64: Probe CPU features via CPUCFG

CPUCFG is a Loongson self-defined instruction used to mark CPU
features for Loongson processors started from Loongson-3A4000.

Slightly adjust cpu_probe_loongson function as well. Remove features
that already probed via decode_configs in processor's PRID case
and add a comment about TLBINV.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>