ARM: dts: keystone: Add ti,keystone-spi for SPI

Add ti,keystone-spi to the compatible field for the SPI node. This new
entry insures that the proper prescaler limit is used for keystone devices

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>