ASoC: remove unnecessary header including in SGTL5000 codec driver

Remove unnecessary headers:
 - mach/hardware.h in sgtl5000.c
 - linux/i2c.h in sgtl5000.h

Signed-off-by: Zeng Zhaoming <b32542@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>