riscv: dts: tenstorrent: Add PMU node to blackhole for Linux perf support

riscv: dts: tenstorrent: Add PMU node to blackhole for Linux perf support

Add a riscv,pmu device tree node with SBI PMU event mappings for the
SiFive X280 hardware performance counters. This enables OpenSBI to
expose the SBI PMU extension, allowing Linux perf to use the 4
programmable counters (mhpmcounter3-6) across 3 event classes:
instruction commit, microarchitectural, and memory system events.

Event encodings are derived from the SiFive Tenstorrent X280 MC Manual
(21G3.04.00) Table 13, section 3.10.5.

Assisted-by: Claude:claude-opus-4-6[1m]
Signed-off-by: Michael Neuling <mikey@neuling.org>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Signed-off-by: Drew Fustini <fustini@kernel.org>