ARM: OMAP: don't build hwspinlock in vain

Cleanup: don't build mach-omap2/hwspinlock.c if the OMAP hwspinlock
driver isn't configured.

This will both shorten build time and avoid registering a device
which isn't needed.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>