Bring in support for single core Zynq devices. Turns out that real
hardware, the registers appear like there's two cores, but the second
core does not work, so base the number of cores upon the chip id.

Tested on a XC7Z007S.

also, previous commit was suppose to be D14429.

Submitted by: Thomas Skibo
Differential Revision: https://reviews.freebsd.org/D14429