UPSTREAM: regulator: core: Add debugfs to show constraint flags

UPSTREAM: regulator: core: Add debugfs to show constraint flags

There are debugfs entries for voltage and current, but not for
the constraint flags. It's useful for debugging to be able to
see what these flags are so this patch adds a new debugfs file.
We can't use debugfs_create_bool for this because the flags are
bitfields, so as this needs a special read callback they have been
collected into a single file that lists all the flags.

Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
(cherry picked from commit 2d80a91b2f2a96f38877bc328dac135d69564911)

Change-Id: Ia3e78960204e34e004340e28b3a7f933aa457371
Signed-off-by: David Wu <david.wu@rock-chips.com>