target/riscv: Ignore reserved bits in PTE for RV64

target/riscv: Ignore reserved bits in PTE for RV64

Highest bits of PTE has been used for svpbmt, ref: [1], [2], so we
need to ignore them. They cannot be a part of ppn.

1: The RISC-V Instruction Set Manual, Volume II: Privileged Architecture
   4.4 Sv39: Page-Based 39-bit Virtual-Memory System
   4.5 Sv48: Page-Based 48-bit Virtual-Memory System

2: https://github.com/riscv/virtual-memory/blob/main/specs/663-Svpbmt-diff.pdf

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Liu Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220204022658.18097-2-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>