soc: cva6: Implement missing cache management APIs

In hardware, cva6 currently only provides global disable/enable
functions for the Dcache and Icache. Disabling and re-enabling them also
has the effect of flushing and invalidating the cache.
Future cva6 SoCs will add support RISC-V's standardized cache management
operations.
This commit provides a default implementation for all methods currently
part of the cache API. These implementations can be overwritten at board
or SoC level, as they use weak linking.

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