dt-bindings: mfd: syscon: Add samsung,exynosautov9-sysreg compatible

Add document Samsung's Exynos Auto v9 compatible for system registers.

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211010032246.146939-2-chanho61.park@samsung.com