powerpc/mpc5xxx: add OF platform binding doc for FSL MSCAN devices

This patch adds documentation for the MSCAN OF device bindings for
the MPC512x and moves the one for the MPC5200 to the new common file
"Documentation/powerpc/dts-bindings/fsl/can.txt".

Signed-off-by: Wolfgang Grandegger <wg@denx.de>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>