dt-bindings: mfd: syscon: Add samsung,exynos850-sysreg

Document Samsung Exynos850 compatible for system registers.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211126230620.478-1-semen.protsenko@linaro.org