net: macsec: documentation for macsec_context and macsec_ops

Add description for fields of struct macsec_context and struct
macsec_ops.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>