dt-bindings: mips: loongson: Add Loongson-1 based boards

Add two Loongson-1 based boards: LSGZ 1B and Smartloong 1C.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>