regulator: Suggest use of datasheet supply or pin names for consumers

Update the documentation to suggest the use of datasheet names for
the supplies requested by regulator consumers. Doing this makes it
easier to tie the design for a given platform up with the requirements
of the driver for a consumer.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>