drm/hdcp: reference for srm file format

In the kernel documentation, HDCP specifications links are shared as a
reference for SRM table format.

v2:
  Fixed small nits. [Shashank]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/320968/?series=57232&rev=14