ARM: ux500: make ux500_cpu_die static

The ux500_cpu_die function is not exported so make it static
to avoid the following warning:

arch/arm/mach-ux500/platsmp.c:89:6: warning: symbol 'ux500_cpu_die' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210323083203.1646842-1-linus.walleij@linaro.org

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>