staging: gasket: Kconfig: describe Apex as an Edge TPU device

Add a brief description and URL for more information on the Apex device,
an Edge TPU (Tensorflow Processing Unit) machine learning accelerator.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>