ARM: dts: imx6ul-var-som: change incorrect VAR-SOM-MX6UL references

ARM: dts: imx6ul-var-som: change incorrect VAR-SOM-MX6UL references

There is no Variscite module named VAR-SOM-MX6UL, but there is VAR-SOM-MX6
and also VAR-SOM-6UL, so it is confusing at first to know to which one it
refers to. The imx6ul-var-som* dts/dtsi supports only the VAR-SOM-6UL [1],
not VAR-SOM-MX6 [2], so modify comments and model descriptions accordingly.

Link  https://dev.variscite.com/var-som-6ul [1]
Link: https://dev.variscite.com/var-som-mx6
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>