regulator: rn5t618: Do not index regulator_desc arrays by id

The regulator_desc arrays in this driver are indexed by RN5T618_*
constants and some elements can be missing. This causes probe failures
on older models:

rn5t618-regulator rn5t618-regulator: failed to register (null) regulator
rn5t618-regulator: probe of rn5t618-regulator failed with error -22

Fix this by making the arrays flat. This also saves a little memory
because the regulator_desc arrays become smaller.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Fixes: 83b2a3c2 ("regulator: rn5t618: add RC5T619 PMIC support")
Signed-off-by: Mark Brown <broonie@kernel.org>