staging,spear_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/staging/iio/adc/spear_adc.c: In function ‘spear_adc_probe’:
drivers/staging/iio/adc/spear_adc.c:393:2: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration

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