target/riscv/cpu.c: validate extensions before riscv_timer_init()

target/riscv/cpu.c: validate extensions before riscv_timer_init()

There is no need to init timers if we're not even sure that our
extensions are valid. Execute riscv_cpu_validate_set_extensions() before
riscv_timer_init().

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230517135714.211809-10-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>