mfd: rk808: Set RK817 interrupt polarity to low

All other rk8xx operate with the polarity on low and even the old
submitted devicetree snippet for the px30-evb declared the irq as low.
So bring the rk817 preset in line with this, as there is really no
reason for it to be the only with with a high polarity.

The rk809/rk817 hasn't been added to any devicetrees so far, so this
won't break anything.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>