boards: openhwgroup: add CVA6 on Genesys 2 board

Adds support for the CVA6 CPU on a Genesys 2 FPGA board
(https://github.com/openhwgroup/cva6

).
The SoC currently contains the CVA6 CPU  with the SV39 MMU, interrupt
controllers (CLINT and PLIC), UART, a SPI for booting from SD, a boot
ROM, and I2C controller for on-board audio, a GPIO and the lowRISC
ethernet subsystem.
Two slightly different versions of the board are added, with a 64-bit
and a 64-bit configuration of CVA6, respectively.

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