Add efidev(4)/efirt(9)

Document efidev(4), provider of userland access to EFI Runtime Services. A link is created to efirtc(4), which handles the time-of-day clock side.

efirt(9) is the kernel side of this.

Reviewed by: imp, kib (earlier version)
Differential Revision: https://reviews.freebsd.org/D16696