arc: add more flavours of ARC700 series CPU

arc: add more flavours of ARC700 series CPU

Now we may select a particular version of ARC700:
 * ARC750D or
 * ARC770D

It allows more flexible (or more fine tuned) configuration of U-Boot.
Before that change we relied on minimal configuration but now we may
use specific features of each CPU.

Moreover allows us to escape manual selection of options that
exist in both CPUs but may have say different version like MMUv2 in
ARC750D vs MMUv3 in ARC770D.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>