target/riscv: Extend PMP region up to 64

target/riscv: Extend PMP region up to 64

According to the RISC-V Privileged Specification (version >1.12),
RV32 supports 16 CSRs (pmpcfg0–pmpcfg15) to configure 64 PMP regions
(pmpaddr0–pmpaddr63).

Signed-off-by: Jay Chang <jay.chang@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250522081236.4050-2-jay.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>