[PowerPC] Fix booke64 qemu infinite loop in L2 cache enable

Since qemu does not implement the L2 cache, we get stuck forever waiting
for a bit to be set when trying to invalidate it.

To prevent that, we should bail out if the L2 cache is missing.
One easy way to check this is L2CFG0 == 0 (since L2CSIZE always has at
least one bit set in a valid implementation)

(tested on qemu, rb800, and x5000)

Reviewed by: jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D25225