RISC-V: Skip setting up PMPs on traps

The RISC-V ISA manual says that PMPs are WARL, but it appears the K210
doesn't implement them and instead traps on the unsupported accesses.
This patch handles those traps by just skipping the PMP
initialization entirely, under the theory that machines that trap on PMP
accesses must allow memory accesses as otherwise they're pretty useless.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>