Disable WPI in case of aml8726-m3.
The aml8726-m3 SoC is identified as a Cortex A9-r2 rev 4 CPU and
it hangs sometimes during the boot when WFI is used by the kernel.

Differential Revision: https://reviews.freebsd.org/D2473
Submitted by: John Wehle
Suggested by: ian@