mfd: axp20x: Mark AXP20X_VBUS_IPSOUT_MGMT as volatile

On AXP288 and newer PMICs, bit 7 of AXP20X_VBUS_IPSOUT_MGMT can be set
to prevent using the VBUS input. However, when the VBUS unplugged and
plugged back in, the bit automatically resets to zero.

We need to set the register as volatile to prevent regmap from caching
that bit. Otherwise, regcache will think the bit is already set and not
write the register.

Fixes: cd532166 ("mfd: axp20x: Fix axp288 volatile ranges")
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>