ARM: at91: sam9260: remove unused platform_data

num_channels and registers are not used anymore since they are defined inside
the at91_adc driver and assigned by matching the id_table.

Also, remove the mach/at91_adc.h include that is not necessary anymore.

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>