arc: add explicit selection of CPU templates ARC 750D and ARC770D

This separation allows to specify unique options and features for each
CPU.

For example ARC 770D has LLOCK/SCOND instructions built-in by default.
Also this new scheme simplifies selection of proper configuration for
users - preconfigured options now match templates for ARC CPUs.

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

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Anton Kolesov <akolesov@synopsys.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>