PowerPC: Add CPLD driver for AmigaOne X5000

Summary:
The CPLD is the communications medium between the CPU and the XMOS
"Xena" event coprocessor. It provides a mailbox communication feature,
along with dual-port RAM to be used between the CPU and XMOS. Also, it
provides basic board stats as well, such as PCIe presence, JTAG signals,
and CPU fan speed reporting (in revolutions per second). Only fan speed
reading is handled, as a sysctl.

Reviewed by: bdragon
Differential Revision: https://reviews.freebsd.org/D23136