parisc: Prepare for Block-TLB support on 32-bit kernel

Change HUGEPAGE_SIZE to become 4 MB on 32-bit kernels, which leads
that kernel code and kernel data will start on 4 MB boundaries.

Although a 32-bit kernel does not support huge pages, most
machines have support for Block-TLBs (BTLB) which allow to
configure the system to use large pages (block TLBs) to minimize
the TLB contention. This is done through calls to PDC and the
32-bit kernel can then call BTLB PDC functions to tell
the machine to optimize the TLBs.

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