Stop using 'magic numbers' when registering interrupt sources.

v2:
Clean redundant comments.
Switch to kernel style comments.

v3:
Add CP_ECC_ERROR define
Signed-off-by: NAndrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>