gpio: em: Use irq_domain_add_simple() to fix runtime error

Adjust the gpio-em.c driver to reconsider the pdata->irq_base
variable. Non-DT board code like for instance board-kzm9d.c
needs to operate of a static IRQ range for platform devices.

So this patch is updating the code to make use of the function
irq_domain_add_simple() instead of irq_domain_add_linear().

Fixes a EMEV2 / KZM9D runtime error caused by the following commit:
7385500a gpio/em: convert to linear IRQ domain

Cc: stable@kernel.org
Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Reported-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>