parisc: use constants instead of numbers in assembly

A few small fixups:
* _PAGE_SIZE_ENCODING_DEFAULT is wrong here, as one might assume that
  it's possible to define the page size that way. This is wrong. Use 0 instead.
* use constants instead of hardcoded numerical values in depi and extru
  while building the PFN out of the pte entry
* use SHRREG instead of extru (iitlba expects the PFN at bits {7..26})

Still wondering why we can use the same register (pte) as extru source
and target register, but it seems to work on PA1.1 and PA2.0...

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>