mmu: rename z_mem_map to z_phys_map

Renamed to make its semantics clearer; this function maps
*physical* memory addresses and is not equivalent to
posix mmap(), which might confuse people.

mem_map test case remains the same name as other memory
mapping scenarios will be added in the fullness of time.

Parameter names to z_phys_map adjusted slightly to be more
consistent with names used in other memory mapping functions.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>