ARM: tango: add Suspend-to-RAM support

Ask firmware to put RAM in self-refresh mode and power system down.

    echo mem > /sys/power/state

See Documentation/power/states.txt

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>