boards: cv*a6: Add twister support

This commit adds the necessary configurations for building and testing
cva6 boards (cv64a6_genesys_2, cv32a6_genesys_2) with twister.
This has been validated against commit
8a9d7a832b7121dd6f9be61a380d1d89ebf2a5f3 of the cva6 hardware project.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>