ARM: plat-versatile: use simple irqdomain for FPGA IRQ

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>