LoongArch: Clarify 3 MSG interrupt features

LoongArch: Clarify 3 MSG interrupt features

LoongArch's MSG interrupt features are used across multiple subsystems.
Clarify these features to avoid misuse, existing users will be adjusted
if necessary.

MSGINT: Infrastructure, means the CPU core supports message interupts.
Indicated by CPUCFG1.MSGINT.

AVECINT: AVEC interrupt controller based on MSGINT, means the CPU chip
supports direct message interrupts. Indicated by IOCSR.FEATURES.DMSI.

REDIRECTINT: REDIRECT interrupt controller based on MSGINT and AVECINT,
means the CPU chip supports redirect message interrupts. Indicated by
IOCSR.FEATURES.RMSI.

For example:
Loongson-3A5000/3C5000 doesn't support MSGINT/AVECINT/REDIRECTINT;
Loongson-3A6000 supports MSGINT but doesn't support AVECINT/REDIRECTINT;
Loongson-3C6000 supports MSGINT/AVECINT/REDIRECTINT.

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