boards: renode: Add virtual Cortex-R8 board

This commit adds a virtual target using the `cortex_r8_virtual` SoC.
It can be used for testing purposes and a starting point to add new R8
platforms.

Signed-off-by: Krzysztof Sychla <ksychla@antmicro.com>
Signed-off-by: Marek Slowinski <mslowinski@antmicro.com>
Signed-off-by: Piotr Zierhoffer <pzierhoffer@antmicro.com>
Signed-off-by: Mateusz Hołenko <mholenko@antmicro.com>