Documentation/gpu: Add fdinfo meanings of panthor-*-memory tags

A previous commit enabled display of driver-internal kernel BO sizes
through the device file's fdinfo interface.

Expand the description of the relevant driver-specific key:value pairs
with the definitions of the new panthor-*-memory ones.

Reviewed-by: Mihail Atanassov <[e-mail]>
Reviewed-by: Liviu Dudau <[e-mail]>
Reviewed-by: Steven Price <[e-mail]>
Signed-off-by: Adrián Larumbe <[e-mail]>
Signed-off-by: Boris Brezillon <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/panthor.rst

···
      drm-cycles-panthor:     94439687187
      drm-maxfreq-panthor:    1000000000 Hz
      drm-curfreq-panthor:    1000000000 Hz
+     panthor-resident-memory:        10396 KiB
+     panthor-active-memory:  10396 KiB
      drm-total-memory:       16480 KiB
      drm-shared-memory:      0
      drm-active-memory:      16200 KiB
···
 
 Where `N` is a bit mask where cycle and timestamp sampling are respectively
 enabled by the first and second bits.
+
+Possible `panthor-*-memory` keys are: `active` and `resident`.
+These values convey the sizes of the internal driver-owned shmem BO's that
+aren't exposed to user-space through a DRM handle, like queue ring buffers,
+sync object arrays and heap chunks. Because they are all allocated and pinned
+at creation time, only `panthor-resident-memory` is necessary to tell us their
+size. `panthor-active-memory` shows the size of kernel BO's associated with
+VM's and groups currently being scheduled for execution by the GPU.