mfd: rk808: Disable the under voltage detect

Rk808 has a under voltage detect function, when the voltage of buck is
under 85% the target voltage, the buck output will reset. But if the
power load is too heavy, this function maybe err, when current over
4.2A, although the voltage is normal, but RK808 mistakenly think it is
under 85%, and reset the buck. So let's disable this function.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>