ASoC: Intel: Baytrail: Fix implicit declaration of function 'memcpy_fromio'

Some kernel configurations can cause following build error:

sound/soc/intel/sst-baytrail-ipc.c: In function ‘sst_byt_get_dsp_position’:
sound/soc/intel/sst-baytrail-ipc.c:744:2: error: implicit declaration of function ‘memcpy_fromio’ [-Werror=implicit-function-declaration]
  memcpy_fromio(&fw_tstamp,
  ^
cc1: some warnings being treated as errors

Fix this by including <linux/io.h> explicitly.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
---
v2: including <linux/io.h> instead of <asm/io.h>. Thanks to Takashi Iwai for
noticing (checkpatch.pl tells it too).

Also sst-dsp., sst-baytrail-dsp.c and sst-haswell-dsp.c are using
memcpy_fromio/_toio but there <asm/io.h> seems to come via
<linux/dma-mapping.h> or <linux/io.h> so I didn't touch them.