regulator: virtual: warn against production use

regulator: virtual: warn against production use

This driver is only meant for debugging and testing.  Currently, it's
not possible to use it without patching the kernel since it requires
platform data, but we'll be adding devicetree support, so add a loud
warning to make it clear that it's still only meant for debugging and
testing.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220301111831.3742383-3-vincent.whitchurch@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>