RISC-V: skip cpu-map when parsing elf_hwcap

The fill_elf_hwcap() function expects to find only cpu nodes under the
/cpus entry of the device tree. Newer versions of QEMU insert a cpu-map
node which describes the CPU topology, breaking this function. To fix
this, simply skip any non-cpu entries.

Reviewed by: markj, kp, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22151