ARM: 7806/1: allow DEBUG_UNCOMPRESS for Tegra

DEBUG_UNCOMPRESS was previously disallowed for Tegra due to tegra.S's
use of global data that was not linked into the decompressor. Solve this
by declaring this symbol in tegra.S when it is being built into the
decompressor. For the kernel proper, leave the declaration in
mach-tegra/common.c as explained in the comment.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>