Documentation/gpu: Add kernel doc entry for MPC

This commit adds a kernel-doc entry for the MPC block. Since it enabled
the kernel-doc to parse some of the documentation in the mpc.h file,
fixing some of the comments was required.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>