ARM: highbank: retry wfi on reset request

In some cases, an interrupt can occur and prevent cause failure to enter
wfi. This causes reset to hang. Retrying the wfi should be enough to
prevent reset from hanging.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Olof Johansson <olof@lixom.net>