gpio/omap: fix off-mode bug: clear debounce settings on free/reset

This change was originally titled "gpio/omap: fix off-mode bug: clear debounce
clock enable mask on free/reset". The title has been updated slightly to
reflect (what should be) the final fix.

When a GPIO is freed or shutdown, we need to ensure that any debounce settings
are cleared and if the GPIO is the only GPIO in the bank that is currently
using debounce, then disable the debounce clock as well to save power.

Currently, the debounce settings are not cleared on a GPIO free or shutdown and
so during a context restore on subsequent off-mode transition, the previous
debounce values are restored from the shadow copies (bank->context.debounce*)
leading to mismatch state between driver state and hardware state.

This was discovered when board code was doing

  gpio_request_one()
  gpio_set_debounce()
  gpio_free()

which was leaving the GPIO debounce settings in a confused state.  If that GPIO
bank is subsequently used with off-mode enabled, bogus state would be restored,
leaving GPIO debounce enabled which then prevented the CORE powerdomain from
transitioning.

To fix this, introduce a new function called _clear_gpio_debounce() to clear
any debounce settings when the GPIO is freed or shutdown. If this GPIO is the
last debounce-enabled GPIO in the bank, the debounce will also be cut.

Please note that we cannot use _gpio_dbck_disable() to disable the debounce
clock because this has been specifically created for the gpio suspend path
and is intended to shutdown the debounce clock while debounce is enabled.

Special thanks to Kevin Hilman for root causing the bug. This fix is a
collaborative effort with inputs from Kevin Hilman, Grazvydas Ignotas and
Santosh Shilimkar.

Testing:
- This has been unit tested on an OMAP3430 Beagle board, by requesting a gpio,
  enabling debounce and then freeing the gpio and checking the register
  contents, the saved register context and the debounce clock state.
- Kevin Hilman tested on 37xx/EVM board which configures GPIO debounce for the
  ads7846 touchscreen in its board file using the above sequence, and so was
  failing off-mode tests in dynamic idle. Verified that off-mode tests are
  passing with this patch.

V5 changes:
- Corrected author

Reported-by: Paul Walmsley <paul@pwsan.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Grazvydas Ignotas <notasas@gmail.com>
Cc: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>