ARM: dts: imx6ull-colibri: use pull-down for adc pins

Disable the keeper and enable a 100k pull-down on the ADC pins as per
the following note in section 13.2 of the i.MX 6ULL Application
Processor Reference Manual, Rev. 1, 11/2017 [1]:

The keeper causes an undesired jump behavior in ADC. To avoid the
problem, disable keeper before starting ADC.

[1] https://www.nxp.com/webapp/Download?colCode=IMX6ULLRM



Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Denys Drozdov <denys.drozdov@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>