Documentation: can: flexcan: Add big-endian property to device tree

The FlexCAN controller can be modelled as little or big endian depending
on SOC design. This device tree property identifies the controller
endianness and the driver reads/writes controller registers based on
that.

This is optional property. i.e. if this property is not present in
device tree node then controller is assumed to be little endian. if this
property is present then controller is assumed to be big endian.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Reviewed-by: Poonam Aggrwal <poonam.aggrwal@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>