ARM: kirkwood: Add standby support

Implements standby support for Kirkwood SoC. When the SoC enters
standby state the memory PM units are disabled, the DDR is set
in self-refresh mode, and the CPU is set in WFI.

At this point there's no clock gating, as that is considered each
driver's task.

Signed-off-by: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>