Add LS1028A clockgen driver

The new driver provides probe and attach functions for the NXP LS1028A
clockgen and passes configuration information to QorIQ clockgen class.

Submitted by: Lukasz Hajec <lha@semihalf.com>
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D30125