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.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

MAINTAINERS

@@ -17948,6 +17948,7 @@ M:	Albert Ou <aou@eecs.berkeley.edu>
 L:	linux-riscv@lists.infradead.org
 S:	Supported
 Q:	https://patchwork.kernel.org/project/linux-riscv/list/
+C:	irc://irc.libera.chat/riscv
 P:	Documentation/riscv/patch-acceptance.rst
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git
 F:	arch/riscv/