drm/panthor: Add uAPI

Panthor follows the lead of other recently submitted drivers with
ioctls allowing us to support modern Vulkan features, like sparse memory
binding:

- Pretty standard GEM management ioctls (BO_CREATE and BO_MMAP_OFFSET),
  with the 'exclusive-VM' bit to speed-up BO reservation on job submission
- VM management ioctls (VM_CREATE, VM_DESTROY and VM_BIND). The VM_BIND
  ioctl is loosely based on the Xe model, and can handle both
  asynchronous and synchronous requests
- GPU execution context creation/destruction, tiler heap context creation
  and job submission. Those ioctls reflect how the hardware/scheduler
  works and are thus driver specific.

We also have a way to expose IO regions, such that the usermode driver
can directly access specific/well-isolate registers, like the
LATEST_FLUSH register used to implement cache-flush reduction.

This uAPI intentionally keeps usermode queues out of the scope, which
explains why doorbell registers and command stream ring-buffers are not
directly exposed to userspace.

v6:
- Add Maxime's and Heiko's acks

v5:
- Fix typo
- Add Liviu's R-b

v4:
- Add a VM_GET_STATE ioctl
- Fix doc
- Expose the CORE_FEATURES register so we can deal with variants in the
  UMD
- Add Steve's R-b

v3:
- Add the concept of sync-only VM operation
- Fix support for 32-bit userspace
- Rework drm_panthor_vm_create to pass the user VA size instead of
  the kernel VA size (suggested by Robin Murphy)
- Typo fixes
- Explicitly cast enums with top bit set to avoid compiler warnings in
  -pedantic mode.
- Drop property core_group_count as it can be easily calculated by the
  number of bits set in l2_present.

Co-developed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
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>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240229162230.2634044-2-boris.brezillon@collabora.com