PM debug: Fix warning when no CONFIG_DEBUG_FS

Fix following warning when CONFIG_DEBUG_FS wasn't selected:

arch/arm/mach-omap2/pm-debug.c:57: warning: 'pm_dbg_init' declared 'static' but never defined

Signed-off-by: Sergio Aguirre <saaguirre@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>