riscv: pmp: work around a QEMU bug

The NAPOT mode isn't computed properly in qemu when the full address
range is covered. Let's hardcode the value that the qemu code checks
explicitly until the appropriate fix is applied to qemu itself.

For reference, here's the qemu patch:
https://lists.gnu.org/archive/html/qemu-devel/2022-04/msg00961.html

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>