ARM: dts: imx6sll-evk: Assign corresponding power supply for vdd3p0

On i.MX6SLL EVK board, sw2 supplies vdd3p0 LDO, this patch assigns
corresponding power supply for vdd3p0 to avoid confusion by below log:

vdd3p0: supplied by regulator-dummy

With this patch, the power supply is more accurate:

vdd3p0: supplied by SW2

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>