of: replace Asahi Kasei Corp vendor prefix

Current vendor-prefixes.txt already has "ak" prefix for Asahi Kasei Corp
by ae8c4209(of: Add vendor prefix for Asahi Kasei Corp.)

It went through the appropriate review process. But, almost all
Asahi Kasei chip drivers are using "asahi-kasei" prefix today.
(arch/arm/boot/dts/tegra20-seaboard.dts only is using "ak,ak8975",
 but there are instances of "asahi-kasei,ak8975" in other dts files.
 And drivers/iio/magnetometer/ak8975.c doesn't support "ak,ak8975" prefix)
So, we made a mistake there.

In addition, checkpatch.pl reports WARNING if it is using "asahi-kasei"
prerfix in DT file.
(DT compatible string vendor "asahi-kasei" appears un-documented)

Marking it deprecated and warning with checkpatch is certainly
preferable. So, this patch replace "ak" to "asahi-kasei" in
vendor-prefixes.txt. (and fixup tegra20-seaboard)

OTOH, Asahi Kasei is usually referred to as "AKM", but this patch
doesn't care about it. Because no DT is using that today.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>