omap2420-h4: Fix booting from NOR flash

The omap2420-h4 board is using a RAM based address as the linker
location for code. This is causing several problems when attempting
to run the latest u-boot code base on this board from flash. Update
the default linker location for code to be in NOR flash. Please note
that OMAP maps the NOR flash to address 0x08000000 by default and so
use this as the default address for the NOR flash.

Also remove legacy code that attempts to calculate where in flash the
sdata structure, that holds the memory interface configuration data,
is located. By changing the default linker location for code to flash
this is no longer necessary.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>