Limit ofw_cpu_early_foreach() to CPUs only

On some platforms, the /cpus node contains cpu-to-cluster
map which deffinitely is not a CPU node. Its presence was
causing incrementing of "id" variable and reporting more
CPUs available than it should.
To make "id" valid, increment it only when an entry really
is a CPU device.

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3216