spi: omap2-mcspi: Select SPI_SPLIT

The function sg_split will be used by spi-omap2-mcspi to handle a SoC
workaround in the SPI driver. Therefore, select SG_SPLIT so this function
is available to the driver.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>