ARM: dts: imx6sl: Add a label for the cpu node

Add a label for the cpu node, so that board devicetree files can
reference to the CPU node.

This is useful for describing a PMIC voltage that supplies the CPU
voltage.

For example:

&cpu0 {
	cpu-supply = <&sw1_reg>;

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>