dt-bindings: mfd: syscon: Add compatibles for Loongson-1 syscon

Add Loongson LS1B and LS1C compatibles for system controller.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>