commit a57f8dac upstream.

The scaling factor for VREFN is 3.0/4096 (not 1.0/4096), just as for
VREFP. This is not immediately obvious from the specification (Xilinx
UG480), but has been confirmed by Xilinx support.
Suggested-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>