ARM: tegra: acer-a500: Remove atmel,cfg_name property

This property was supposed to be upstreamed, but it was NAKed recently
in a favor to a better approach of firmware loading. It also turned
out that the firmware loading isn't really necessary because it's stored
in a non-volatile memory inside of the touchscreen controller and
previously the FW loading was needed in order to get touchscreen working,
but it actually was a TS driver problem which is resolved now. Hence
remove the unsupported property.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>