mx28evk: Provide default values for SPI bus and chip select

Provide default values for SPI bus and chip select.

This allows the command "sf probe" to work without passing SPI bus and chip
select numbers as arguments.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>