drm/panthor: Use a local iomem base for MMU AS registers

drm/panthor: Use a local iomem base for MMU AS registers

Add an MMU_AS_CONTROL local iomem pointer to struct panthor_mmu and
switch AS register accesses to that base.

Interrupt accesses remain routed through the IRQ-local iomem base, while
the MMU register definitions are adjusted so AS registers are expressed
relative to the local MMU AS window. This completes the conversion away
from using the global device mapping for MMU AS register accesses.

No functional change intended.

v3:
- Pick up R-bs from Liviu and Steve
v2:
- Pick up Ack from Boris.

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Karunika Choo <karunika.choo@arm.com>
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260427155934.416502-9-karunika.choo@arm.com