staging: regulator: hi6421v600-regulator: better handle modes

Instead of implementing a custom set of properties, set
valid_modes_mask based on having or not a mask for enabling
the eco_mode.

This makes the code clearer, and remove some uneeded props
from DT.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0f4ac5a4167bbca428c8507b6992acd8b3ebabd1.1597647359.git.mchehab+huawei@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>