regulator: fan53555: Add support for FAN53526

The FAN53526 differs from the FAN53555 only in that the mode bit in
VSEL0/VSEL1 is moved to the CONTROL register, the voltage selector mask
is extended by 1 bit and the step is different.

So extend the existing fan53555 driver to support FAN53526 as well.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>