dts: bindings: can: rename nxp,kinetis-flexcan binding to nxp,flexcan

Rename the nxp,kinetis-flexcan devicetree compatible to nxp,flexcan as it
is not specific to the NXP Kinetis series.

This is preparation for adding a nxp,flexcan-fd binding.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>