can: mcp251x: remove deprecated board file setup example

In the pre device-tree ARM aera there were board files that configured
the system (instead of a device tree). A "struct spi_board_info" was
used to describe the SPI bus.

As new systems should be described via device trees, this patch removes
the board setup example from the driver. The "struct
mcp251x_platform_data" cannot be removed completely, as there are still
some in-tree users of this file.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>