iio: at91: Use different prescal, startup mask in MR for different IP

For at91 boards, there are different IPs for adc. Different IPs has different
STARTUP & PRESCAL mask in ADC_MR.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>