drm/panthor: Use a local iomem base for firmware control registers

drm/panthor: Use a local iomem base for firmware control registers

Add an MCU_CONTROL-local iomem pointer to struct panthor_fw and use it
for firmware control and status register accesses.

Job interrupt accesses continue to go through the IRQ-local base, while
doorbell writes stay on the device-wide mapping because they live
outside the MCU control window. This keeps firmware register accesses
scoped to the component that owns them.

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/20260427155934.416502-8-karunika.choo@arm.com