ASoC: samsung idma: Add proper annotation for casting iomem pointers

It is not always possible to interchange iomem pointers with normal pointers,
which why we have annotations for iomem pointers and warn when casting them to a
normal pointer or vice versa. In this case the casting is fine and unfortunately
necessary so add the proper annotations to tell code checkers that it is
intentional. This silences the following warnings from sparse:
	sound/soc/samsung/idma.c:354:20: warning: incorrect type in argument 1
	 (different address spaces) expected void volatile [noderef]
	  <asn:2>*addr got unsigned char *area
	sound/soc/samsung/idma.c:372:22: warning: cast removes address space of
	 expression

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>