parisc: allow to build with 16k default kernel page size

Introduce new convert_for_tlb_insert20 macro and use it to replace assembler
statements with hardcoded constants.
This change allows the parisc64 kernel to boot with 16kb default kernel page size,
aka CONFIG_PARISC_PAGE_SIZE_16KB=y.

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