ARM: imx6: rename pm-imx6q.c to pm-imx6.c

The pm-imx6q.c works for all i.MX6 SoCs, so let's rename it to pm-imx6.c
and have the build controlled by option CONFIG_SOC_IMX6.

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