ARM: AT91: Add platform data for the AT91 ADCs

The AT91 SoCs often embeds an ADC. This patch adds the needed
platform data to specify the informations required by the driver
to work properly.

For now, we only need the reference voltage and which channels
are available on the board.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>