ARM: OMAP: McSPI: Remove unused flag from struct omap2_mcspi_device_config

Flag single_channel in struct omap2_mcspi_device_config is not used
by drivers/spi/spi-omap2-mcspi.c so we may remove it from include/plat/mcspi.h
and affected board files.

Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>