regulator: rk808: Lower log level on optional GPIOs being not available

RK808 can leverage a couple of GPIOs to tweak the ramp rate during DVS
(Dynamic Voltage Scaling). These GPIOs are entirely optional but a
dev_warn() appeared when cleaning this driver to use a more up-to-date
gpiod API. At least reduce the log level to 'info' as it is totally
fine to not populate these GPIO on a hardware design.

This change is trivial but it is worth not polluting the logs during
bringup phase by having real warnings and errors sorted out
correctly.

Fixes: a13eaf02

 ("regulator: rk808: make better use of the gpiod API")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20191203164709.11127-1-miquel.raynal@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>