soc: renode: Add cortex_r8_virtual

Add virtual Cortex R8 SoC. This target does not represent a real SoC,
but can be easily run in Renode.

This will allow to easily test basic architecture support.

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>