support both direct and indirect accessor in unified
helper functions.

v2: Retire indirect mmio access via mm_index/data
Signed-off-by: NHawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NKevin Wang <kevin1.wang@amd.com>
Reviewed-by: NGuchun Chen <guchun.chen@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>