mfd: Implement tps6586x gpio_to_irq

The TPS6586x adds the interrupt of this device using
linear mapping on irq domain.
Hence, implement gpio_to_irq to get the irq number
corresponding to TPS6586x GPIOs which is created
dynamically.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>