This switches the FPGA IRQ driver over to using the simple IRQ
domain. We can then use the same codepath for this in the
DT and non-DT cases.

To be able to use quicker irq_find_mapping() in the handlers
we first call irq_create_mapping() on all the valid HW IRQ
numbers so that descriptors will be created for them in the
DT case where a linear domain will be the outcome of the
call.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>