gpio: omap: convert to use generic irq handler

gpio: omap: convert to use generic irq handler

This patch converts TI OMAP GPIO driver to use generic irq handler
instead of chained IRQ handler. This way OMAP GPIO driver will be
compatible with RT kernel where it will be forced thread IRQ handler
while in non-RT kernel it still will be executed in HW IRQ context.
As part of this change the IRQ wakeup configuration is applied to
GPIO Bank IRQ as it now will be under control of IRQ PM Core during
suspend.

There are also additional benefits:
 - on-RT kernel there will be no complains any more about PM runtime usage
   in atomic context  "BUG: sleeping function called from invalid context";
 - GPIO bank IRQs will appear in /proc/interrupts and its usage statistic
    will be  visible;
 - GPIO bank IRQs could be configured through IRQ proc_fs interface and,
   as result, could be a part of IRQ balancing process if needed;
 - GPIO bank IRQs will be under control of IRQ PM Core during
   suspend to RAM.

Disadvantage:
 - additional runtime overhed as call chain till
   omap_gpio_irq_handler() will be longer now
 - necessity to use wa_lock in omap_gpio_irq_handler() to W/A warning
   in handle_irq_event_percpu()
   WARNING: CPU: 1 PID: 35 at kernel/irq/handle.c:149 handle_irq_event_percpu+0x51c/0x638()

This patch doesn't fully follows recommendations provided by Sebastian
Andrzej Siewior [1], because It's required to go through and check all
GPIO IRQ pin states as fast as possible and pass control to handle_level_irq
or handle_edge_irq. handle_level_irq or handle_edge_irq will perform actions
specific for IRQ triggering type and wakeup corresponding registered
threaded IRQ handler (at least it's expected to be threaded).
IRQs can be lost if handle_nested_irq() will be used, because excecution
time of some pin specific GPIO IRQ handler can be very significant and
require accessing ext. devices (I2C).

Idea of such kind reworking was also discussed in [2].

[1] http://www.spinics.net/lists/linux-omap/msg120665.html
[2] http://www.spinics.net/lists/linux-omap/msg119516.html

Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Austin Schuh <austin@peloton-tech.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>