regulator: tps65917/palmas: Handle possible memory allocation failure

Stop the palmas regulator driver from imagining that the allocations
will always succeed. Since regulator dt nodes are optional in nature and
can be described in downstream drivers via platform data, continue to
maintain code flow as prior when of node is not found.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>