ARM: dts: am335x: Use correct vendor prefix for Asahi Kasei Corp.

The old "ak" vendor prefix that was never officially accepted was still
being used in some device trees. Convert to the correct vendor prefix
(i.e. "asahi-kasei").

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>