mfd: Move twl-core IRQ allocation into twl[4030|6030]-irq files

During DT adaptation, the irq_alloc_desc was added into twl-core, but
due to the rather different and weird IRQ management required by the twl4030,
it is much better to have a different approach for it.
The issue is that twl4030 uses a two level IRQ mechanism but handles all the
PWR interrupts as part of the twl-core interrupt range. It ends up with a
range of 16 interrupts total for CORE and PWR.

The other twl4030 functionalities already have a dedicated driver and thus
their IRQs and irqdomain can and should be defined localy.

twl6030 is using a single level IRQ controller and thus does not require any
trick.

Move the irq_alloc_desc and irq_domain_add_legacy in twl4030-irq and
twl6030-irq.

Allocate together CORE and PWR IRQs for twl4030-irq.

Conflicts:

        drivers/mfd/twl-core.c

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>