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>
diff --git a/MAINTAINERS.yml b/MAINTAINERS.yml
index aa4fbaa..f03d141 100644
--- a/MAINTAINERS.yml
+++ b/MAINTAINERS.yml
@@ -2281,6 +2281,7 @@
     - katsuster
     - edersondisouza
     - carlocaione
+    - npitre
   files:
     - arch/riscv/
     - boards/riscv/