omap: mcbsp: Drop SPI mode support

We haven't seen any use for the SPI API in McBSP driver over the years. More
over, Peter Ujfalusi <peter.ujfalusi@ti.com> noticed that SPI mode is not
even supported since OMAP2430 so it's very unlikely that we'll see any use
for it in the future either.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>