ARM: 8177/1: cacheflush: Fix v7_exit_coherency_flush exynos build breakage on ARMv6

This fixes build breakage of platsmp.c if ARMv6 was chosen for compile
time options (e.g. by building allmodconfig):

$ make allmodconfig
$ make
  CC      arch/arm/mach-exynos/platsmp.o
/tmp/ccdQM0Eg.s: Assembler messages:
/tmp/ccdQM0Eg.s:432: Error: selected processor does not support ARM mode `isb '
/tmp/ccdQM0Eg.s:437: Error: selected processor does not support ARM mode `isb '
/tmp/ccdQM0Eg.s:438: Error: selected processor does not support ARM mode `dsb '
make[1]: *** [arch/arm/mach-exynos/platsmp.o] Error 1

The error was introduced in commit "ARM: EXYNOS: Move code from
hotplug.c to platsmp.c".  Previously code using
v7_exit_coherency_flush() macro was built with '-march=armv7-a' flag but
this flag dissapeared during the movement.

Fix this by annotating the v7_exit_coherency_flush() asm code with
armv7-a architecture.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reported-by: Mark Brown <broonie@kernel.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>