dt-bindings: mips: Document two Loongson boards

Document loongson64c-4core-ls7a and loongson64g-4core-ls7a, two boards
with LS7A PCH.

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