dt-bindings: reset: npcm: add GCR syscon property

Describe syscon property that handles general control registers (GCR) in
Nuvoton BMC NPCM reset driver.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>