Since in the DT case, the linear domain path will not allocate
descriptors for the IRQs, we need to use irq_create_mapping()
for mapping hwirqs to Linux IRQs, so these descriptors get
created on-the-fly in this case.

ChangeLog v1->v2:

- Just use irq_create_mapping() in the .to_irq function since
  this is called before unmasking or enabling any interrupt
  lines, so irq_find_mapping() should be sufficient for the
  IRQ handler function.

Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>