parisc: Fix pmd code to depend on PT_NLEVELS value, not on CONFIG_64BIT

Make the code which sets up the pmd depend on PT_NLEVELS == 3, not on
CONFIG_64BIT. The reason is, that a 64bit kernel with a page size
greater than 4k doesn't need the pmd and thus has PT_NLEVELS = 2.

Signed-off-by: Helge Deller <deller@gmx.de>