m28: Properly configure the SPI flash chipselect

The SPI flash is not properly detected by plain "sf probe" due to
it being located on different bus and different chipselect. Fix
this problem.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>