dt-bindings: ufs: samsung,exynos-ufs: Add size constraints on "samsung,sysreg"

The 'phandle-array' type is a bit ambiguous. It can be either just an
array of phandles or an array of phandles plus args. "samsung,sysreg" is
the latter and needs to be constrained to a single entry with a phandle and
offset.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240124190733.1554314-1-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>