platform/x86/amd: amd_3d_vcache: Add sysfs ABI documentation

platform/x86/amd: amd_3d_vcache: Add sysfs ABI documentation

Add documentation for the amd_3d_vcache sysfs bus platform driver
interface so that userspace applications can use it to change mode
preferences, either frequency or cache.

Co-developed-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Perry Yuan <perry.yuan@amd.com>
Co-developed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Reviewed-by: Armin Wolf <W_Armin@gmx.de>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Basavaraj Natikar <Basavaraj.Natikar@amd.com>
Link: https://lore.kernel.org/r/20241112170307.3745777-3-Basavaraj.Natikar@amd.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>