regulator: core: Optimise enable/disable path for always on regulators

regulator: core: Optimise enable/disable path for always on regulators

If a regulator is always on for any reason then cache that when the
consumer is created and use it to optimise away the need to take locks
or recurse up the supply tree when consumers do enable or disable calls.
The scheduling of asynchronous work for bulk enables is also skipped.

We don't actually check if the device physically supports control on the
basis that constraints allowing status changes on physically always on
regulators are nonsensical anyway.

This is a very common pattern in hardware - it's normal to have some
power supplies that have either no software control or are critical to
system function - so many systems should be able to benefit.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>