sh_eth: add R-Car support for real

Commit d0418bb7123f44b23d69ac349eec7daf9103472f (net: sh_eth: Add eth support
for R8A7779 device) was a failed attempt to add support for one of members of
the R-Car SoC family.  That's for three reasons: it treated R8A7779 the  same
as SH7724 except including quite dirty hack adding ECMR_ELB  bit  to the mask
in sh_eth_set_rate() while not removing ECMR_RTM bit (despite it's reserved in
R-Car Ether), and it didn't add a new register offset array despite the closest
SH_ETH_REG_FAST_SH4 mapping differs by 0x200 to the offsets all the R-Car Ether
registers have, and also some of the registers in this old mapping don't exist
on R-Car Ether (due to this, SH7724's 'sh_eth_my_cpu_data' structure is not
adequeate for R-Car too).  Fix all these shortcomings, restoring the SH7724
related section to its pristine state...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>