Call devmap_bootstrap in RISC-V machine dependent code to actually create
the static device mappings.
While RISC-V support was added to subr_devmap.c in r298631, it was never
actually initialised in the machine dependent code.
Submitted by: Nicholas O'Brien <nickisobrien_gmail.com>
Reviewed by: br, kp
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D21975