The imx5x and imx6 chips have an onboard IOMUX device which also contains a
few "general purpose registers" whose values control chip behavior in ways
that have nothing to do with IO pin mux control.  Define a simple API that
other soc-specific code can use to read and write the registers, and provide
the imx51 implementation of them.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271084 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f