regulator: Update DA9121 dt-bindings

Update bindings for the Dialog Semiconductor DA9121 voltage regulator to
add device variants.
Because several variants have multiple regulators, and to regard potential
to add GPIO support in future, the 'regulators' sub-node is added,
following the precedent set by other multi-regulator devices, including
the DA9211 family. This breaks compatibility with the original submission
by Vincent Whitchurch - but as this is still in for-next, the alignment
could be made before upstreaming occurs.

Signed-off-by: Adam Ward <Adam.Ward.opensource@diasemi.com>
Link: https://lore.kernel.org/r/0606d3ded5fef4c38760246146f197db4ce3a374.1606755367.git.Adam.Ward.opensource@diasemi.com
Signed-off-by: Mark Brown <broonie@kernel.org>