MAINTAINERS: Add myself as a riscv reviewer

MAINTAINERS: Add myself as a riscv reviewer

The goal is for me to get a kernel.org account and then send pull requests
in order to relieve some pressure from Palmer and make our workflow
smoother.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Enthusiastically-Supported-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20241212131134.288819-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>