drivers: adc: mcux_adc12: use DT_INST defines for instances

Convert the NXP Kinetis ADC12 driver from relying on CONFIG_ADC_n
Kconfig defines to using DT_INST defines for instance configuration.

This resolves the issue of having e.g. ADC12 instances 2 and 3
enabled, but not instance 0.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>