MAINTAINERS: Update mailing list for Renesas ARM64 SoC Development

Update the mailing list used for development of support for ARM64
Renesas SoCs.

This is a follow-up for a similar change for other Renesas SoCs and
drivers uses by Renesas SoCs.  The ARM64 SoC entry was not updated in
that patch as it was not yet present in mainline.

The motivation for the mailing list update is that Renesas SoCs are now
much wider than the SH architecture and there is some desire from some
for the linux-sh list to refocus on discussion of the work on the SH
architecture.

Acked-by: Magnus Damm <damm@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>