xtensa: simplify coherent_kvaddr logic

Functions coherent_kvaddr, clear_page_alias and copy_page_alias use
physical address 0 as a special value that means 'this page is in the
KSEG mapping and its existing virtual address has the same color as the
virtual address of its future mapping, so don't map it to the
TLBTEMP_BASE area'.

Simplify this logic and drop special handling of low memory pages/pages
with coherent mapping and always use TLBTEMP_BASE area.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>