ARM: ux500: Staticise ux500_soc_attr

This variable is not used outside of the file that it's declared in, so
reduce the scope to the local file.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[Also add const marker after suggestion from Lee Jones]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>