Merge branch 'Documentation-stmmac-documentation-improvements'

Jose Abreu says:

====================
Documentation: stmmac documentation improvements

Converts stmmac documentation to RST format.

1) Adds missing entry of stmmac documentation to MAINTAINERS.

2) Converts stmmac documentation to RST format and adds some new info.

3) Adds the new RST file to the list of files.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>