Fix boot up on ARMADA38X uniprocessor variant

Marvell Armada 380 is a uni-processor variant of the 38x SoC
family. A function platform_mp_setmaxid() was setting a hardcoded
value, which caused boot fail on A380. Fix this by relying on
the CPU count obtained from device tree nodes.

Submitted by: Marcin Wojtas <mw@semihalf.com
Obtained from: Semihalf
Sponsored by: Netgate
Reviewed by: loos
Differential revision: https://reviews.freebsd.org/D10783