ARM: kirkwood: kirkwood_pm_init() should return void

This function was originally meant to return void as declared in the
common.h header. Fix it and include the header to catch these errors
in the future.

[jac] removed 'return 0;' to clear this warning:

  arch/arm/mach-kirkwood/pm.c: In function 'kirkwood_pm_init':
  arch/arm/mach-kirkwood/pm.c:73:2: warning: 'return' with a value, in function returning void [enabled by default]

Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>