soc: add OpenHW Group CVA6 SoC

Adds support for the CVA6 family of RISC-V CPUs.
CVA6 is commonly found as a soft core CPU on FPGA designs.
Different configurations and instruction set extensions can be
configured, and different SoCs targeting various FPGA boards are
available.
This commit adds support for the 32-bit and 64-bit configurations
of CVA6, as well as three slightly different SoCs (a minimal 32-bit
configuration, a 64-bit configuration without FPU, a 64-bit
configuration with FPU).

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