mfd: 88pm800: 88pm805: Remove "IRQF_TRIGGER_FALLING" flag

88pm800/88pm805 interrupt is asserted low if the events happened.
So remove IRQF_TRIGGER_FALLING for irq request.
Also, the interrupt wiring is board dependent so do not set
IRQF_TRIGGER by default.

Signed-off-by: Yi Zhang <yizhang@marvell.com>
Signed-off-by: Chao Xie <chao.xie@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>