regulator: core: support shared enable GPIO concept

regulator: core: support shared enable GPIO concept

 A Regulator can be enabled by external GPIO pin.
 This is configurable in the regulator_config.
 At this moment, the GPIO can be owned by only one regulator device.
 In some devices, multiple regulators are enabled by shared one GPIO pin.
 This patch extends this limitation, enabling shared enable GPIO of regulators.

 New list for enable GPIO: 'regulator_ena_gpio_list'
   This manages enable GPIO list.

 New structure for supporting shared enable GPIO: 'regulator_enable_gpio'
   The enable count is used for balancing GPIO control count.
   This count is incremented when GPIO is enabled.
   On the other hand, it's decremented when GPIO is disabled.

 Reference count: 'request_count'
   The reference count, 'request_count' is incremented/decremented on
   requesting/freeing the GPIO. This count makes sure only free the GPIO
   when it has no users.

 How it works
   If the GPIO is already used, skip requesting new GPIO usage.
   The GPIO is new one, request GPIO function and add it to the list of
   enable GPIO.
   This list is used for balancing enable GPIO count and pin control.

 Updating a GPIO and invert code moved
   'ena_gpio' and 'ena_gpio_invert' of the regulator_config were moved to
    new function, regulator_ena_gpio_request().
    Use regulator_enable_pin structure rather than regulator_dev.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>