riscv: pmp: work around another QEMU bug

A QEMU bug may create bad transient PMP representations causing
false access faults to be reported. Work around it by setting
pmp registers to zero from the update start point to the end
before updating them with new values.

The QEMU fix is here with more details about this bug:
https://lists.gnu.org/archive/html/qemu-devel/2022-06/msg02800.html



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