ARM: imx: clk-imx6q: AXI clock select index is incorrect

The AXI clock mux should be as below:

00: periph;
01: pll2_pfd2_396m;
10: periph;
11: pll3_pfd1_540m;

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>