target/riscv: add PTE_A/PTE_D/PTE_U bits check for inner PTE

For non-leaf PTEs, the D, A, and U bits are reserved for future standard use.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220204022658.18097-3-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>