davinci: add spi devices support for MityDSP-L138/MityARM-1808 platform

This patch adds support for accessing the on board SPI NOR FLASH
device for MityDSP-L138 and MityARM-1808 SoMs.

Signed-off-by: Michael Williamson <michael.williamson@criticallink.com>
Tested-by: Michael Williamson <michael.williamson@criticallink.com>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>