gpio: omap: Remove custom PM calls and use cpu_pm instead

For a long time the gpio-omap custom PM calls have been annoying me so
let's replace them with cpu_pm instead. This will enable GPIO PM for
deeper idle states on omap4. And we can handle GPIO PM for omap2/3/4
in the same way.

Note that with this patch we are also slightly changing GPIO PM to be
less aggressive for omap3 and only will idle GPIO when PER context
may be lost.

For omap2, we don't need to save context and don't want to remove any
triggering so let's add a quirk flag for that.

Let's do this all in a single patch to avoid a situation where old
custom calls still are used with new code.

Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Keerthy <j-keerthy@ti.com>
Cc: Ladislav Michl <ladis@linux-mips.org>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>