[SPARC64]: Do not allocate OBP page tables using bootmem

Just allocate them physically starting from the end of
the kernel image.  This incredibly simplifies our MM
bootstrap in that we don't need any mappings in the linear
PAGE_OFFSET area working in order to bootstrap ourselves and
take over the trap table from the firmware.

Many further simplifications are possible now, and this also
sets the stage for CONFIG_DEBUG_PAGEALLOC support.

Signed-off-by: David S. Miller <davem@davemloft.net>