gpio: mpc8xxx: Fix support for IRQ_TYPE_LEVEL_LOW flow_type in mpc85xx

Commit e39d5ef6 ("powerpc/5xxx: extend mpc8xxx_gpio driver to support
mpc512x gpios") implemented support for IRQ_TYPE_LEVEL_LOW flow type in
mpc512x via falling edge type. Do same for mpc85xx which support was added
in commit 345e5c8a ("powerpc: Add interrupt support to mpc8xxx_gpio").

Fixes probing of lm90 hwmon driver on mpc85xx based board which use level
interrupt. Without it kernel prints error and refuse lm90 to work:

    [   15.258370] genirq: Setting trigger mode 8 for irq 49 failed (mpc8xxx_irq_set_type+0x0/0xf8)
    [   15.267168] lm90 0-004c: cannot request IRQ 49
    [   15.272708] lm90: probe of 0-004c failed with error -22

Fixes: 345e5c8a ("powerpc: Add interrupt support to mpc8xxx_gpio")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>