ARM: 6431/1: fix isb regression on CPU < v7

The kernel does not compile for my ARM926EJ-S system U300 due to
the isb instruction inserted in generic assember statement from
commit 8925ec4c, "ARM: 6385/1:
setup: detect aliasing I-cache when D-cache is non-aliasing"
hey the isb is only available when assembling for v7 so let's
use the generic isb() macro from setup.h instead.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>