Build modules specific to imx5/imx6 only when building those kernels.

This adds sys/modules/imx with a SUBDIR makefile to make the whole
collection of modules that are specific to these SoCs. Initially, that
"whole collection" consists of the if_ffec and imx_i2c drivers.

The if_ffec driver is referenced in its existing home in ../ffec rather
than moving it into the imx directory, because it's used by powerpc too,
but it is no longer built for all armv6/7 systems.

The imx_i2c driver is newly added as a module.