ARM: imx6q: Add GPR3 MIPI muxing control register field shift bits definition

This patch adds a macro to define the GPR3 MIPI muxing control register field
shift bits.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>