perf vendor events riscv: Rename U74 to Bullet

perf vendor events riscv: Rename U74 to Bullet

This set of PMU event descriptions applies not only to the SiFive U74
core configuration, but also to other SiFive cores that implement the
Bullet microarchitecture (such as U64, P270, and X280). Rename the
directory to be more generic.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Tested-by: Ian Rogers <irogers@google.com>
Tested-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20250213220341.3215660-2-samuel.holland@sifive.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>