MAINTAINERS: declare myself as a RISC-V collaborator

... given that 62.5% of the code in arch/riscv/core/* is mine.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

MAINTAINERS.yml

@@ -2281,6 +2281,7 @@ RISCV arch:
     - katsuster
     - edersondisouza
     - carlocaione
+    - npitre
   files:
     - arch/riscv/
     - boards/riscv/