ARM: rmobile: Set maximum kernel size to 64 MiB on Gen3

The Gen3 kernel images are often above 8 MiB, increase the
maximum kernel size to 64 MiB to future-proof it, just like
many other ARM64 boards do.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>