sys: Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Convert the imx6_snvs RTC driver to access registers via the syscon device.
This is required for it to work correctly in the GENERIC kernel.

Make the imx6_snvs driver usable as a module, add pnp info. Add a manpage.