iio: adc: ep93xx: Enable wider build testing with COMPILE_TEST

It is useful to be able to build this driver without needing
to build support for ARCH_EP93XX.

Also add an explicit dependency on HAS_IOMEM so we have
devm_platform_ioremap_resource() etc available.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Link: https://lore.kernel.org/r/20221231144128.489810-1-jic23@kernel.org