iio: adc: Kconfig: Append vendor name for IMX7D_ADC

To keep IMX7D_ADC text consistent with other iio/adc entries, add
the vendor name to the driver menu text.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>