NOTE: THIS ONLY WORKS WHEN KERNEL RUNS IN M-MODE FOR NOW
This frees the PMP for other use, e.g. HART memory separation.
The page tables are statically allocated, 1 per level.
This feature is now behind CONFIG_MPFS_USE_MMU_AS_MPU, because
only the MPFS target supports this (others are not tested).
If the MMU is used for memory separation within a HART, the PMP must
still be configured to allow user access to the memory mapped for the
HART, because PMP *rekoves* access by default. At this point all of
the user memory as well as the kernel RAM are opened.
A more flexible solution for PMP configuration will follow.
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