dts: bindings: add devicetree bindings for CAN transceivers

Add generic devicetree bindings for simple CAN transceivers.

Always-on CAN transceivers are considered passive and just provide a
maximum supported bitrate.

Active CAN controllers can typically be controlled by the MCU via either
SPI, I2C, or GPIO. Common GPIO controlled CAN transceivers provide
either a stand-by or an enable pin (or both) for controlling the state
of the CAN transceiver.

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