ARM: OMAP: OMAP_DEBUG_LEDS needs to select LEDS_CLASS

This fixes below build error when CONFIG_LEDS_CLASS is not set.

  LD      init/built-in.o
arch/arm/plat-omap/built-in.o: In function `fpga_probe':
arch/arm/plat-omap/debug-leds.c:113: undefined reference to `led_classdev_register'
make: *** [vmlinux] Error 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>