1) Removed request_irq() and replaced it with request_threaded_irq().
2) Removed generic_handle_irq() and replaced it with
handle_nested_irq().
Handling of these interrupts is nested, as we are handling an
interrupt (for e.g rtc, mmc1) when we are still servicing TWL irq.
3) Removed I2C read-retry logic for the case when twl_i2c_read() is
failed inside IRQ handler - there is no sense to do that, so just report
an error and return.
4) Each nested IRQ is configured with corresponding parent_irq,
which need to be retriggered in case if nested IRQ is marked
as IRQS_PENDING.