gpio/em: convert to linear IRQ domain

gpio/em: convert to linear IRQ domain

The code in the em driver seems to want to try to do the job of
the linear IRQ domain (allocate descriptors and grab a virtual
range). So why not just use the linear IRQ domain? The code is
now cut down so we don't need isolated functions for this.

Also note that we use irq_create_mapping() to make sure
descriptors are allocated for these IRQs.

Also fixed the FIXME to remove the domain after use.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Magnus Damm <damm@opensource.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>