ARM: mvebu: compile pm code conditionally

A cleanup to include the headers correctly caused another build problem:

arch/arm/mach-mvebu/kirkwood-pm.c:70:13: error: redefinition of 'kirkwood_pm_init'
arch/arm/mach-mvebu/kirkwood-pm.h:23:20: note: previous definition of 'kirkwood_pm_init' was here

The underlying issue is that kirkwood-pm.o is not actually meant to be
used when CONFIG_PM is disabled, so we should also leave it out of the
Makefile.

The same seems to be true for the PM code in MACH_MVEBU_V7, and I'm
treating it the same way here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: d705c1a6 ("ARM: Kirkwood: fix kirkwood_pm_init() declaration/type")
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>