GPIO: mvebu-gpio: Don't initialize the mask_cache

Due to the SMP nature of some of the chips, which have per CPU
registers, the driver does not use the generic irq_gc_mask_set_bit() &
irq_gc_mask_clr_bit() functions, which only support a single register.
The driver has its own implementation of these functions, which can
pick the correct register depending on the CPU being used. The
functions do however use the gc->mask_cache value.

The call to irq_setup_generic_chip() was passing
IRQ_GC_INIT_MASK_CACHE, which caused the gc->mask_cache to be
initialized to the contents of some random register. This resulted in
unexpected interrupts been delivered from random GPIO lines.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Jamie Lentin <jm@lentin.co.uk>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>