parisc: Drop PMD_SHIFT from calculation in pgtable.h

PMD_SHIFT isn't defined if CONFIG_PGTABLE_LEVELS == 3, and as
such the kernel test robot found this warning:

 In file included from include/linux/pgtable.h:6,
                  from arch/parisc/kernel/head.S:23:
 arch/parisc/include/asm/pgtable.h:169:32: warning: "PMD_SHIFT" is not defined, evaluates to 0 [-Wundef]
     169 | #if (KERNEL_INITIAL_ORDER) >= (PMD_SHIFT)

Avoid the warning by using PLD_SHIFT and BITS_PER_PTE.

Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: kernel test robot <lkp@intel.com>
Cc: <stable@vger.kernel.org> # 6.0+