mfd: twl6040: Correct Ready and Thermal interrupt handling

Create new irq handler for thermal events in order to be able to handle the
event and clean up the code regarding to interrupt handling:
Use proper function names for the irq handlers
No need to read the INTD register anymore.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>