target/riscv: Avoid reporting odd-numbered pmpcfgX in the CSR XML for RV64

target/riscv: Avoid reporting odd-numbered pmpcfgX in the CSR XML for RV64

At present the odd-numbered PMP configuration registers for RV64 are
reported in the CSR XML by QEMU gdbstub. However these registers do
not exist on RV64 so trying to access them from gdb results in 'E14'.

Move the pmpcfgX index check from the actual read/write routine to
the PMP CSR predicate() routine, so that non-existent pmpcfgX won't
be reported in the CSR XML for RV64.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230228104035.1879882-11-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>