ARM: dts: imx6sl-evk: Remove incorrect power supply assignment

ARM: dts: imx6sl-evk: Remove incorrect power supply assignment

The vdd3p0 LDO's input should be from external USB VBUS directly, NOT
PMIC's power supply, the vdd3p0 LDO's target output voltage can be
controlled by SW, and it requires input voltage to be high enough, with
incorrect power supply assigned, if the power supply's voltage is lower
than the LDO target output voltage, it will return fail and skip the LDO
voltage adjustment, so remove the power supply assignment for vdd3p0 to
avoid such scenario.

Fixes: 3feea8805d6f ("ARM: dts: imx6sl-evk: Assign corresponding power supply for LDOs")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>