overo: add SPL support

* implementation based on ti beagleboard/omap3evm
* timing data and i2c workaround for revision 0 boards taken from x-loader
* run-tested with overo revision 0 and 1 / boot from NAND and SDcard
* run-tested with x-loader

Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Signed-off-by: Tom Rini <trini@ti.com>