greybus: lights: enable multi color LED support

A backport (commit 79c4de08c0e5a26b04a4ac9e6543dad6379f0b40) was
applied in kernel which adding attribute-group support in led-class.
Remove the LED_HAVE_GROUPS flag check entirely that allow led
drivers to create custom attributes like color/fade_in/fade_out.

Testing Done: Compiled and verified on EVT2 and gpbridge-test module
              with device class daughter board.

Signed-off-by: Kris Huang <huang_kris@projectara.com>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>