dm: regulator: add function device_get_supply_regulator()

dm: regulator: add function device_get_supply_regulator()

Some devices are supplied by configurable regulator's output.
But there was no function for getting it. This commit adds
function, that allows for getting the supply device by it's phandle.

The returned regulator device can be used with regulator uclass's API.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>