ASoC: atmel_pcm: Remove non-existant header

<mach/hardware.h> doesn't exist on AVR32 and therefore this driver won't
build on that arch.  AFAICT this driver doesn't actually use the content
of that header so easiest just to remove it.

Signed-off-by: Ben Nizette <bn@niasdigital.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>