mfd: Convert tps6586x to irq_domain

Allocate the irq base if it base is not porvided i.e.
in case of device tree invocation of this driver.
Convert the tps6586x driver to irq domain, using a
legacy IRQ mapping if an irq_base is specified in
platform data or dynamically allocated and otherwise
using a linear mapping.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>