iio: adc: at91: cleanup platform_data

num_channels and registers are not used anymore since they are defined inside
the driver and assigned by matching the id_table.

Also, struct at91_adc_reg_desc is now only used inside the driver.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>