gpio/mvebu: Use irq_domain_add_linear

This fixes the irq allocation in this driver to not print:
 irq: Cannot allocate irq_descs @ IRQ34, assuming pre-allocated
 irq: Cannot allocate irq_descs @ IRQ66, assuming pre-allocated

Which happens because the driver already called irq_alloc_descs()
and so the change to use irq_domain_add_simple resulted in calling
irq_alloc_descs() twice.

Modernize the irq allocation in this driver to use the
irq_domain_add_linear flow directly and eliminate the use of
irq_domain_add_simple/legacy

Fixes: ce931f57 ("gpio/mvebu: convert to use irq_domain_add_simple()")
Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>