MAINTAINERS: Specify IRC channel for Renesas ARM64 port

The Renesas ARM ports do have their own IRC channel #renesas-soc (initially
created on Freenode, then moved to Liberta.Chat).  Hopefully, adding it to
this file will attract more people... :-)

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Link: https://lore.kernel.org/r/6c08e98f-c7bb-9d95-5032-69022e43e39b@omp.ru


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>