ASoC: SIU driver shall select FW_LOADER

The SIU ASoC driver must load firmware to program the DSP, therefore it
has to select FW_LOADER in its Kconfig entry.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>