iio:ad5064: Structural changes to support LTC2617

This patch makes minor structural changes to support specifics
for LTC2617 DAC. This DAC requires different handling of the
power down modes. The configuration to actually support the
DAC will be submitted in a secondary patch.

Adjust the DECLARE_AD5064_CHANNELS() macro to accept a new
ext_info parameter. This allows to use different power down
modes per DAC. (e.g. DAC only support 90kohm to ground)

Add the chip_info parameter "powerdown_ltc". This parameter is
used in the ad5064_sync_powerdown_mode() function to handle the
power down command for LTC diffently. For those devices the
power down command must be addressed to the channel.

Signed-off-by: Marc Andre <marc.andre@netline.ch>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>