ARM: dts: imx7-colibri: add basic supply regulators

Colibri modules need to be powered using the power pins 3V3 and
AVDD_AUDIO. Add fixed regulators which represent this power rails.
Potentially, those power rails could be switched on a carrier
board. A carrier board device tree could add a own regulator with
a GPIO, and reference that regulator in a vin-supply property of
those new module level system regulators. This also synchronize
the name of the +3.3V regulator with the one used in the Colibri
VF50/VF61 device tree.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>