Add QorIQ platform clockgen driver.

This patch adds classes and functions that can be used with various NXP
QorIQ Layerscape SoCs.

As for the clock topology - there is single platform PLL, which supplies
clocks for the peripheral bus and additional PLLs for CPU cores. There
can be multiple core PLLs (For example - LS1046A has two PLLs - CGAPLL1
and CGAPLL2). Each PLL has fixed dividers on output. The core PLLs
are not accessible from dts.

This is a preparation patch for NXP LS1046A SoC support.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: mmel
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential Revision: https://reviews.freebsd.org/D24351