ARM: imx6: drop .text.head section annotation from headsmp.S

The function v7_secondary_startup() works just fine in .text section, so
there is no need to have .text.head section annotation at all.  Drop it.

Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>