ARM: kirkwood: Give pm.c its own header file.

The pm code needs to be separated from common.h in order to split DT
and non-DT systems apart. Move the declarations into a header file of
its own and include it where needed.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com> (on kirkwood)
Signed-off-by: Jason Cooper <jason@lakedaemon.net>