ARM: dts: imx6sx-sabreauto: drop board specific pinctrl container node

The board specific pinctrl function container node 'imx6x-sabreauto' is
not really necessary, since commit 5fcdf6a7 ("pinctrl: imx: Allow
parsing DT without function nodes").  Let's drop it to save one level of
indentation.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>