remove memory domain restriction on system RAM for memory partitions on MMU devices

[andrewboie]

Is your enhancement proposal related to a problem? Please describe.
MMU devices have a documented restriction that k_mem_partitions must be within system RAM bounds in order to be programmed into a memory domain.

This is overly restrictive. We should allow k_mem_partition to be installed for any mapped memory address, including virtual mappings from z_mem_map().

The immediate use-case is allowing for user-mode drivers that can be granted access to MMIO regions.

Describe the solution you'd like
Documentation removed that memory domain partitions can only be SRAM address ranges.
x86 implementation updated to support this.
TBD: can we provide a generic test?