ARM: oxnas: make ox820_boot_secondary static

Fix sparse warning:

arch/arm/mach-oxnas/platsmp.c:30:12: warning: symbol 'ox820_boot_secondary' was
not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ma Feng <mafeng.ma@huawei.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Fixes: af76e806 ("ARM: oxnas: Add OX820 SMP support")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/1589247973-29728-1-git-send-email-mafeng.ma@huawei.com