gpio: Provide the STMPE GPIO driver with its own IRQ Domain

The STMPE GPIO driver can be used as an IRQ controller by some
related devices. Here we provide it with its very own IRQ Domain
so that IRQs can be issued dynamically. This will stand the
driver in good stead when it is enabled for Device Tree, as this
it a prerequisite.

Cc: Grant Likely <grant.likely@secretlab.ca>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>