[PARISC] Fix BLK_BOUNCE_HIGH on parisc by initializing max_low_pfn

max_low_pfn was not being set in arch/parisc/mm/init.c, causing severe
problems whenever anything tried to use BLK_BOUNCE_HIGH. Set it to
max_pfn like other similar architectures do.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>