Use devm_mfd_add_devices() for adding MFD child devices and
devm_request_threaded_irq() for IRQ registration.

This reduces the need of remove callback for removing MFD child
devices and unregistering IRQ.
Signed-off-by: NLaxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>