MIPS: Loongson: Add comments for interface_info

Clarify meanings of members of interface_info, especially for .size,
which does not define the ROM size.

Suggested-by: Icenowy Zheng <uwu@icenowy.me>
Signed-off-by: Kexy Biscuit <kexybiscuit@aosc.io>
Acked-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>