target/riscv: Fix the PMP is locked check when using TOR

target/riscv: Fix the PMP is locked check when using TOR

The RISC-V spec says:
    if PMP entry i is locked and pmpicfg.A is set to TOR, writes to
    pmpaddri-1 are ignored.

The current QEMU code ignores accesses to pmpaddri-1 and pmpcfgi-1 which
is incorrect.

Update the pmp_is_locked() function to not check the supporting fields
and instead enforce the lock functionality in the pmpaddr write operation.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 2831241458163f445a89bd59c59990247265b0c6.1618812899.git.alistair.francis@wdc.com