ARM: dts: imx6sl-evk: make pfuze100 sw4 always on

On i.MX6SL EVK board, pfuze100 sw4 supplies
LPDDR2 which is critical for system, must be
always on.

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