OMAP3: PM debug: fix build error when !CONFIG_DEBUG_FS

The PM debug code fails to build on when CONFIG_DEBUG_FS is not
enabled.

Build error log:
arch/arm/mach-omap2/built-in.o: In function `omap_sram_idle':
arch/arm/mach-omap2/pm34xx.c:449: undefined reference to `pm_dbg_regset_save'
arch/arm/mach-omap2/pm34xx.c:460: undefined reference to `pm_dbg_regset_save'
arch/arm/mach-omap2/built-in.o: In function `configure_vc':
arch/arm/mach-omap2/pm34xx.c:1237: undefined reference to `pm_dbg_regset_init'
arch/arm/mach-omap2/pm34xx.c:1238: undefined reference to `pm_dbg_regset_init'
make: *** [.tmp_vmlinux1] Error 1

This patch fixes the above errors.

Kernel booting is tested on omap zoom2 and zoom3 boards.

Signed-off-by: Manjunatha GK <manjugk@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>