omap5912-osk: Fix booting from NOR flash

The omap5912-osk 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 at address
0x00000000.

The omap5912-osk board only has 32MB of RAM and so fix the comment
in the omap5912-osk config.mk file as well.

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