ARM: OMAP2+: PM: fix wakeupgen warning when hotplug disabled

When CONFIG_HOTPLUG_CPU=n, there are unused functions in wakeupgen:

arch/arm/mach-omap2/omap-wakeupgen.c:181: warning: 'wakeupgen_irqmask_all' defined but not used

Fix this by moving all the functions only used when CONFIG_HOTPLUG_CPU=y
together and wrapping in an #ifdef.

No functional changes.

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>