ARM: imx6q: use common soc revision helpers

It calls imx_set_soc_revision() to set up soc revision in
imx6q_init_revision(), and replaces all the occurrences of
imx6q_revision() with common helper imx_get_soc_revision().

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