ARM: dts: phycore-imx6: set buck regulator modes explicitly

This patch sets initial buck regulator modes explicitly to a state this
hardware needs. So a wrong initial mode set by bootloader or pmic itself
does not interfere anymore.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>