mfd: axp20x: Correct AXP806 POK interrupt prefix

When AXP806 support was added, POK was incorrectly expanded to PWROK.
However, the datasheet lists them as POK[LSNP], which is the same as
on the AXP288. Furthermore, the registers associated with POK functions
are the same as the PEK on the other AXP PMICs. This suggests that
"POK" means "Power On Key", much like "PEK" means "Power Enable Key",
instead of "Power OK".

This patch changes the "PWROK" prefix to "POK" for these interrupts.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>