MAINTAINERS: add an IRC entry for RISC-V

I remember being told "Just ping me on IRC" about patches, but googling
at the time was not helpful. #riscv on libera is not linux specific,
but a bunch of contributors etc do hang out there.
Add a link to the maintainers entry to help others find it in the future!

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230106125344.1685266-1-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>