regulator: Add (devm_)of_regulator_get()

The Rockchip power-domain controller also plans to make use of
per-domain regulators similar to the MediaTek power-domain controller.
Since existing DTs are missing the regulator information, the kernel
should fallback to the automatically created dummy regulator if
necessary. Thus the version without the _optional suffix is needed.

The Rockchip driver plans to use the managed version, but to be
consistent with existing code the unmanaged version is added at the
same time.

Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>