parisc: Replace PT_NLEVELS with CONFIG_PGTABLE_LEVELS

The following warning is seen when compiling parisc images

./arch/parisc/include/asm/pgalloc.h: In function 'pgd_alloc':
./arch/parisc/include/asm/pgalloc.h:29:5: warning: "PT_NLEVELS" is not defined

Some definitions of PT_NLEVELS were missed with the conversion to
CONFIG_PGTABLE_LEVELS.

Fixes: f24ffde4

 ("parisc: expose number of page table levels
	on Kconfig level")
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Helge Deller <deller@gmx.de>