regulator: Add empty devm_regulator_bulk_get for !CONFIG_REGULATOR

This fixes below build error if CONFIG_REGULATOR is disabled.

  CC      sound/soc/codecs/wm5100.o
sound/soc/codecs/wm5100.c: In function ‘wm5100_i2c_probe’:
sound/soc/codecs/wm5100.c:2462: error: implicit declaration of function ‘devm_regulator_bulk_get’
make[3]: *** [sound/soc/codecs/wm5100.o] Error 1
make[2]: *** [sound/soc/codecs] Error 2
make[1]: *** [sound/soc] Error 2
make: *** [sound] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>