This switches the legacy irqdomain to the simple one, which will
auto-allocate descriptors, and also make sure that we use
irq_create_mapping() in the to_irq function. Also use the map function
of irq_domain_ops to setup the irq configuration on demand and no more
statically during the initialization of the driver.

Based on a initial patch from Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>