gpio/mvebu: convert to use irq_domain_add_simple()

The MVEBU driver probably just wants a few IRQs. Using the simple
domain has the upside of allocating IRQ descriptors if need be,
especially in a SPARSE_IRQ environment.

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>