parisc: map_pages(): cleanup page table initialization

The current code uses '#if PTRS_PER_PMD == 1' to distinguish 2 vs 3 levels,
setup, it casts pgd to pgd to cope with page table folding and converts
addresses of page table entries from physical to virtual and back for no
good reason.

Simplify the accesses to the page table entries using proper unfolding of
the upper layers and replacing '#if PTRS_PER_PMD' with explicit
'#if CONFIG_PGTABLE_LEVELS == 3'

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Helge Deller <deller@gmx.de>