regulator: of: fix suspend-min/max-voltage parsing

Currently the regulator-suspend-min/max-microvolt must be within the
root regulator node but the dt-bindings specifies it as subnode
properties for the regulator-state-[mem/disk/standby] node. The only DT
using this bindings currently is the at91-sama5d2_xplained.dts and this
DT uses it correctly. I don't know if it isn't tested but it can't work
without this fix.

Fixes: f7efad10

 ("regulator: add PM suspend and resume hooks")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://lore.kernel.org/r/20190917154021.14693-3-m.felsch@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>