ARM: Kirkwood: fix kirkwood_pm_init() declaration/type

ARM: Kirkwood: fix kirkwood_pm_init() declaration/type

The kirkwood-pm.c was missing the include of kirkwood-pm.h to
define the kirkwood_pm_init() function. However once this is
included, the types do not match.

Fixup the include, and then the prototype to avoid the following
warning:

arch/arm/mach-mvebu/kirkwood-pm.c:69:12: warning: symbol 'kirkwood_pm_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>