target/riscv/pmp: Convert qemu_log_mask(LOG_TRACE) to trace events

Use the always-compiled trace events, remove the now unused
RISCV_DEBUG_PMP definition.

Note pmpaddr_csr_read() could previously do out-of-bound accesses
passing addr_index >= MAX_RISCV_PMPS.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>