ARM: OMAP: Correct save_boot_params and replace with 'C' function

Currently save_boot_params saves the boot parameters passed
from romcode. But this is not stored in a writable location
consistently. So the current code would not work for a
'XIP' boot. Change this by saving the boot parameters in
'gd' which is always writable. Also add a 'C' function
instead of an assembly code that is more readable.

Signed-off-by: Sricharan R <r.sricharan@ti.com>