Currentlly, we use Synopsys DesignWare DMA Controller for
baytrail/haswell/broadwell ADSP firmware loading, but for
skylake, we don't use it, compiling sst-firmware.c may
introduce error when CONFIG_DW_DMAC_CORE is not enabled:

    sound/built-in.o: In function `sst_dma_new':
    (.text+0xd7b38): undefined reference to `dw_dma_probe'
    sound/built-in.o: In function `sst_dma_free':
    (.text+0xd7c0a): undefined reference to `dw_dma_remove'

Here we only compile sst-firmware when CONFIG_DW_DMAC_CORE
is selected, to fix the linking error issue.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jie Yang <yang.jie@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>