spmi: pmic-arb: Remove custom show_resume_irq code

The generic irq layer supports showing the resume irq via
/sys/power/pm_wakeup_irq as of commit a6f5f0dd ("PM / sleep:
Report interrupt that caused system wakeup"). Remove the custom
code here that duplicates that code.

Change-Id: Ic955890cf5d80743429f908a2be7489b4d84b15e
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>