SPL: Add NOR flash booting support

SPL NOR flash booting support is quite simple. Only copying of the
images is needed.

On MPC5xxx we need to make sure to only use the standard memcpy()
implementation and not the MPC5xxx specific one. As the MPC5xxx
version has some complexity which is not needed for this SPL
booting.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@ti.com>