staging:iio:mxs-lradc: Select STMP_DEVICE

The MXS ADC driver uses the stmp_reset_block() which is only provided when the
STMP_DEVICE Kconfig symbol is selected. Hence the driver should select this
symbol. So far this has not been a problem since the driver depends on ARCH_MXS,
which already selects STMP_DEVICE, but will become necessary once we allow the
driver to be built when COMPILE_TEST is selected.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>