ARM: dts: imx6sl: vddpu is NOT an always-on regulator

Remove "regulator-always-on" property for vddpu regulator
since it can be OFF when GPU power domain is OFF.

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