gpio: omap: Block idle on pending gpio interrupts

With the SoC cpuidle handling fixed for cpu_pm, we can now start to
return NOTIFY_BAD if there there are pending gpio interrupts.

This way the deeper SoC idle states can get blocked, and gpio latency
is improved in some cases. Note that this will not help with the
latency if the SoC has already entered a deeper idle state.

Note that this patch depends on cpu_pm properly handling the errors
returned by notifiers. For omap variants, this is fixed with patch
"ARM: OMAP2+: Handle errors for cpu_pm".

Cc: Dave Gerlach <d-gerlach@ti.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Cc: Ladislav Michl <ladis@linux-mips.org>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20200304225433.37336-3-tony@atomide.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>