Documentation/gpu: Add acronyms for some firmware components

Users can check the file "/sys/kernel/debug/dri/0/amdgpu_firmware_info"
to get information on the firmware loaded in the system. This file has
multiple acronyms that are not documented in the glossary. This commit
introduces some missing acronyms to the AMD glossary documentation. The
meaning of each acronym in this commit was extracted from code
documentation available in the following files:

- drivers/gpu/drm/amd/amdgpu/gfx_v7_0.c
- drivers/gpu/drm/amd/include/amd_shared.h

Changes since v1:
- Expand acronym meanings based on Alex Deucher suggestions.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>