MAINTAINERS: Add RISC-V maintainers

Add risc-v maintainers and update collaborators.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>
Signed-off-by: Karol Gugala <kgugala@antmicro.com>
Signed-off-by: Peter Gielda <pgielda@antmicro.com>
Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>
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