omap: GPIO module enable/disable

This patch disables a GPIO module when all pins of a GPIO
module are inactive (clock gating forced at module level) and
enables the module when any gpio in the module is requested.

The module is enabled only when "mod_usage" indicates that no GPIO
in that  module is currently active and the GPIO being requested
is the 1st one to be active in that module.

Each module would be disabled in omap_gpio_free() API when all
GPIOs in a particular module becomes inactive. The module is
re-enabled in omap_gpio_request() API when a GPIO is requested
from the module that was previously disabled.

Since individual GPIO's bookkeeping is added in this patch
via "mod_usage", the same is used in omap_set_gpio_debounce()
& omap_set_gpio_debounce_time() APIs to ensure that the gpio being
used is actually "requested" prior to being used (Nishant Menon's
<nm@ti.comSuggestion)

GPIO module level details are specific to hardware and hence
introducing this patch in low level layer (plat-omap/gpio.c)

Signed-off-by: Charulatha V <charu@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>