Add LS1046A clockgen driver.

Driver provides probe and attach functions for LS1046A clockgen and passes
configuration information to QorIQ clockgen class. It may be used as
a reference implementation for different QorIQ clockgen devices.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: mmel, manu
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D24352