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.
The old implementation used the default ld.script for the kernel side
which did not obey the memory.ld limits whatsoever.
Also, provide the user space addresses from the linker script to get rid
of the pre-processor macros that define (incorrect) default values for
the user space composition.
