dt-bindings: sh_eth convert bindings to json-schema

Convert Renesas Electronics SH EtherMAC bindings documentation to
json-schema.  Also name bindings documentation file according to the compat
string being documented.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>