gpio: mvebu: clear irq in edge cause register before unmask edge irq

gpio: mvebu: clear irq in edge cause register before unmask edge irq

When input GPIO set from 0 to 1, the interrupt bit asserted in the GPIO
Interrupt Cause Register (ICR) even if the corresponding interrupt
masked in the GPIO Interrupt Mask Register.

Because interrupt mask register only affects assertion of the interrupt
bits in Main Interrupt Cause Register and it does not affect the
setting of bits in the GPIO ICR.

So, there is problem, when we unmask interrupt with already
asserted bit in the GPIO ICR, then false interrupt immediately occurs
even if GPIO don't change their value since last unmask.

Signed-off-by: Maxim Kiselev <bigunclemax@gmail.com>
Link: https://lore.kernel.org/r/20200115073811.24438-1-bigunclemax@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>