staging,lpc32xx_adc: Add dependency on HAS_IOMEM

On archs like S390 or um this driver cannot build nor work.
Make it depend on HAS_IOMEM to bypass build failures.

drivers/built-in.o: In function `lpc32xx_adc_probe':
drivers/staging/iio/adc/lpc32xx_adc.c:149: undefined reference to `devm_ioremap'

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: David S. Miller <davem@davemloft.net>