pata_arasan: add Device Tree probing capability

SPEAr platforms now support DT and so must convert all drivers to support DT.
This patch adds DT probing support for Arasan Compact Flash controller and
updates its documentation too.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>