arm: mach-omap2: am33xx: Add device structure for spi

Add platform data and a device structure for the spi device
present on am335x-icev2. This requires moving all omap3_spi
platform data structures and symbols to an omap3_spi.h so that
the board file can access them.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>