iio: adc: at91-sama5d2_adc: convert to platform specific data structures

Convert the driver to platform specific structures. This means:
- create a register layout struct that will hold offsets for registers
- create a platform struct that will hold platform information (number of
channels, indexes for different channels and pointer to layout struct)
- convert specific macros that are platform dependent into platform variables

This step is in fact a no-op, but allows the driver to be more flexible
and for future enhancement including adding new platforms that are partly
compatible with the current driver and differ slightly in register layout
or capabilities for example.
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lore.kernel.org/r/20210901123013.329792-5-eugen.hristev@microchip.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>