ARM: tegra: acer-a500: Correct compatible of ak8975 magnetometer

The "ak,ak8975" compatible is not recognized by dt-bindings, it's
deprecated. Use supported "asahi-kasei,ak8975" compatible.

Signed-off-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Thierry Reding <treding@nvidia.com>