ARM: dts: imx51: drop "fsl,imx-osc"

"fsl,imx-osc" is useless since no driver deals with it, so let's drop it.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>