ARM: spear: rename duplicate pl080_plat_data

Both spear3xx and spear6xx have a global symbol named
pl080_plat_data. Eventually, both should be removed, but
for now, we can rename one to pl080_plat_data and declare
it static, since that one does not actually need to be
visible outside of spear6xx.c.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>