hw/riscv, target/riscv: move pmu fdt function to fdt-common.c

riscv_pmu_generate_fdt_node() can be moved to fdt_common.c since it has
no PMU TCG internals.

Signed-off-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Message-ID: <20260703180538.3346781-4-daniel.barboza@oss.qualcomm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>