arm: Tegra2: Fix ELDK42 gcc failure with inline asm stack pointer load

The 4.2.2 gcc in the ELDK42 release doesn't like the direct SP
load using a constant in tegra2_start. Change it to use a load
thru another reg using mov sp, %0 : : "r"(CONST).

Tested on my Seaboard T20-A03, U-Boot loads and runs OK. Also
compiled all tegra2 builds with both gcc 4.2.2 and 4.4.1 OK.

Signed-off-by: Tom Warren <twarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>