riscv: look for bootargs in FDT

The FDT may contain a short /chosen/bootargs string which we should pass
to boot_parse_cmdline. Notably, this allows the use of qemu's -append
option to pass things like -s to boot to single user mode.

Submitted by: Nathaniel Filardo <nwf20@cl.cam.ac.uk>
Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25544