mfd: Simplify IRQ domain registration code in STMPE

Historically, a driver would have to decide whether it required
a Linear or Legacy IRQ domain when registering one. This can end
up as quite a lot of code. A new Simple call now exists which
simplifies this process. Let's make use of it here.

Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>