dt-bindings: mfd: syscon: Merge Samsung Exynos Sysreg bindings

The Samsung Exynos System Registers (Sysreg) bindings are quite simple -
just additional compatible to the syscon.  They do not have any value so
merge them into generic MFD syscon bindings.

Suggested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>