iio: imx8qxp-adc: fix dependency to the intended ARCH_MXC config

Commit 1e23dcaa ("iio: imx8qxp-adc: Add driver support for NXP IMX8QXP
ADC") adds the config IMX8QXP_ADC for this new driver, which depends on
the non-existing config ARCH_MXC_ARM64.

Hence, ./scripts/checkkconfigsymbols.py warns:

  ARCH_MXC_ARM64
  Referencing files: drivers/iio/adc/Kconfig

Probably, the existing config ARCH_MXC is intended to be referred here.
So, repair the dependency to refer to that config.

Fixes: 1e23dcaa

 ("iio: imx8qxp-adc: Add driver support for NXP IMX8QXP ADC")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20211111083954.6286-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>