riscv: Small fix to CPU compatibility identification

fdt_is_compatible_strict() inspects the first compatible property.
We need to inspect the following properties for 'riscv'.
ofw_bus_node_is_compatible() does a recursive search.

This patch fixes "Can't find CPU" error message when bootverbose = true.

Submitted by: Nicholas O'Brien (nickisobrien_gmail.com)
Reviewed by: philip, kp
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D21576