ARM: dts: imx6sl-evk: Assign corresponding power supply for LDOs

On i.MX6SL EVK board, sw2 supplies vdd1p1/vdd2p5/vdd3p0 LDO, this
patch assigns corresponding power supply for vdd1p1/vdd2p5/vdd3p0
to avoid confusion by below log:

vdd1p1: supplied by regulator-dummy
vdd3p0: supplied by regulator-dummy
vdd2p5: supplied by regulator-dummy

With this patch, the power supply is more accurate:

vdd1p1: supplied by SW2
vdd3p0: supplied by SW2
vdd2p5: supplied by SW2

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