ARM: dts: imx6ull-dhcor: Set and limit the mode for PMIC buck 1, 2 and 3

According to Renesas Electronics (formerly Dialog Semiconductor), the
standard AUTO mode of the PMIC DA9061 can lead to stability problems
depending on the hardware revision. It is recommended to set a defined
mode such as PFM or PWM permanently. So set and limit the mode for
buck 1, 2 and 3 to a fixed one.

Fixes: 611b6c89 ("ARM: dts: imx6ull-dhcom: Add DH electronics DHCOM i.MX6ULL SoM and PDK2 board")
Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>