drm/panthor: Fix firmware initialization on systems with a page size > 4k

The system and GPU MMU page size might differ, which becomes a
problem for FW sections that need to be mapped at explicit addresses
since our PAGE_SIZE alignment might cover a VA range that's
expected to be used for another section.

Make sure we never map more than we need.

Changes in v3:
- Add R-bs

Changes in v2:
- Plan for per-VM page sizes so the MCU VM and user VM can
  have different pages sizes

Fixes: 2718d918 ("drm/panthor: Add the FW logical block")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241030150231.768949-1-boris.brezillon@collabora.com