MAINTAINERS: add RISCV related directories

This patch gathers and adds all RISCV related directories into
'RISCV arch' entry.
It is preliminary modification until suitable maintainer (or entry)
for each RISCV boards or sub-architecture appears in the future.

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

MAINTAINERS.yml

@@ -987,7 +987,10 @@ RISCV arch:
         - katsuster
     files:
         - arch/riscv/
+        - boards/riscv/
+        - dts/bindings/riscv/
         - include/arch/riscv/
+        - soc/riscv/
     labels:
         - "area: RISCV"