sh_eth: add sh_eth_cpu_data::gecmr flag

Not all Ether controllers having the Gigabit register layout have GECMR --
RZ/A1 (AKA R7S72100) actually has the same layout but no Gigabit speed
support and hence no GECMR. In the past, the new register map table was
added for this SoC, now I think we should have used the existing Gigabit
table with the differences (such as GECMR) covered by the mere flags in
the 'struct sh_eth_cpu_data'. Add such flag for GECMR -- and then we can
get rid of the R7S72100 specific layout in the next patch...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Tested-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>