dt-bindings: timer: Add andestech atcpit100 timer binding doc

Add a document to describe Andestech atcpit100 timer and
binding information.

Signed-off-by: Rick Chen <rickchen36@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>