85xx: MP Boot Page Translation update

This change has 3 goals:
- Have secondary cores be released into spin loops at their 'true'
  address in SDRAM.  Previously, secondary cores were put into spin
  loops in the 0xfffffxxx address range which required that boot page
  translation was always enabled while cores were in their spin loops.

- Allow the TLB window that the primary core uses to access the
  secondary cores boot page to be placed at any address.  Previously, a
  TLB window at 0xfffff000 was always used to access the seconary cores'
  boot page.  This TLB address requirement overlapped with other
  peripherals on some boards (eg XPedite5370).  By default, the boot
  page TLB will still use the 0xfffffxxx address range, but this can be
  overridden on a board-by-board basis by defining a custom
  CONFIG_BPTR_VIRT_ADDR.  Note that the TLB used to map the boot page
  remains in use while U-Boot executes.  Previously it was only
  temporarily used, then restored to its initial value.

- Allow Boot Page Translation to be disabled on bootup.  Previously,
  Boot Page Translation was always left enabled after secondary cores
  were brought out of reset.  This caused the 0xfffffxxx address range
  to somewhat "magically" be translated to an address in SDRAM.  Some
  boards may not want this oddity in their memory map, so defining
  CONFIG_MPC8xxx_DISABLE_BPTR will turn off Boot Page Translation after
  the secondary cores are initialized.

These changes are only applicable to 85xx boards with CONFIG_MP defined.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>