MAINTAINERS: Add myself to RISCV subsys collaborators.

Currently there is no maintainer and active collaborator in RISCV
subsys area. I hope this helps to encourage RISCV developments.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>

MAINTAINERS.yml

@@ -1008,6 +1008,7 @@ RISCV arch:
     collaborators:
         - mgielda
         - nategraff-sifive
+        - katsuster
     files:
         - arch/riscv/
         - include/arch/riscv/