MAINTAINERS: Specify IRC channel for Renesas ARM32 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/2f108f63-0cf7-cc4c-462e-ec63736234cf@omp.ru


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