soc: samsung: exynos-regulator-coupler: Add simple voltage coupler for Exynos5800

Add a simple custom voltage regulator coupler for Exynos5800 SoCs, which
require coupling between "vdd_arm" and "vdd_int" regulators. This coupler
ensures that the voltage values don't go below the bootloader-selected
operation point during the boot process until the clients set their
constraints. It is achieved by assuming minimal voltage value equal to
the current value if no constraints are set. This also ensures proper
voltage balancing if any of the client driver is missing.

The balancing code comes from the regulator/core.c with the additional
logic for handling regulators without client constraints applied added.

Link: https://lore.kernel.org/r/20200721180900.13844-5-krzk@kernel.org
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>