ARM: 6895/1: mach-ux500: fix SMP secondary startup regression

Commit e2a083dc0da9aa6437e14811198379b18cdfa7f8
"ARM: consolidate SMP cross call implementation" broke the ux500
compilation since the smp.h header declared a function called
from headsmp.S. This fixes it up by declaring it locally instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>