iio: adc: mxs-lradc: fix non-static symbol warnings

Fix sparse warning "symbol foo was not declared. Should it be static?"
for the following symbols:
mx23_lradc_adc_irq_names
mx28_lradc_adc_irq_names
iio_dev_attr_in_voltage0_scale_available
iio_dev_attr_in_voltage1_scale_available
iio_dev_attr_in_voltage2_scale_available
iio_dev_attr_in_voltage3_scale_available
iio_dev_attr_in_voltage4_scale_available
iio_dev_attr_in_voltage5_scale_available
iio_dev_attr_in_voltage6_scale_available
iio_dev_attr_in_voltage7_scale_available
iio_dev_attr_in_voltage10_scale_available
iio_dev_attr_in_voltage11_scale_available
iio_dev_attr_in_voltage12_scale_available
iio_dev_attr_in_voltage13_scale_available
iio_dev_attr_in_voltage14_scale_available
iio_dev_attr_in_voltage15_scale_available

Signed-off-by: Paolo Cretaro <paolocretaro@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>