MAINTAINERS: Add unowned RISC-V related files to the right sections

MAINTAINERS: Add unowned RISC-V related files to the right sections

There are a bunch of RISC-V files that are currently not covered
by the "get_maintainers.pl" script. Add them to the right sections
in MAINTAINERS to fix this problem.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christoph Müllner <christoph.muellner@vrull.eu>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>