arm: drop volatile qualifier from gd pointer

arm: drop volatile qualifier from gd pointer

There's a bunch of other places where this qualifier should be
dropped, e.g. in the set_gd() prototype and for various variables used
for stashing the value in the mach-imx/ directory and elsewhere. But
that will be done in follow-up patches.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Tested-by: Anshul Dalal <anshuld@ti.com>