ARM: imx6: fix v7_invalidate_l1 by adding I-Cache invalidation

The recent suspend/resume and reset testing on imx6q discovers that
not only D-Cache but also I-Cache has random data and validity when
the core comes out of a power recycle.

This patch adds I-Cache invalidation into v7_invalidate_l1 to make
sure both D-Cache and I-Cache invalidated on power-up.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>