drm/panthor: Store IRQ register base iomem pointer in panthor_irq

drm/panthor: Store IRQ register base iomem pointer in panthor_irq

Update common IRQ handling code to work from an IRQ-local iomem base
instead of referencing block-specific interrupt register offsets.

Store the interrupt base address iomem pointer in struct panthor_irq and
switch the shared IRQ helpers to use generic INT_* offsets from that
local base. This removes the need for each caller to expose absolute IRQ
register addresses while keeping the common IRQ flow unchanged.

No functional change intended.

v3:
- Clean up definition of pwr->irq.iomem.
v2:
- Change IRQ request function to accept an iomem pointer instead of
  computing it from an offset argument.

Signed-off-by: Karunika Choo <karunika.choo@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260427155934.416502-5-karunika.choo@arm.com