powerpc/85xx: allow CONFIG_PHYS_64BIT to be selectable

Remove the "select PHYS_64BIT" from the Kconfig entry for the P1022DS,
so that large physical address support is a selectable option for non-CoreNet
reference boards.

The option is enabled in mpc85xx_[smp_]defconfig so that the default is
unchanged.  However, now it can be deselected.

The P1022DS had this option defined because the default device tree for
this board uses 36-bit addresses.  This had the side-effect of forcing
this option on for all boards that use mpc85xx_[smp_]defconfig.  Some
users may want to disable this feature to create an optimized configuration
for boards with <= 2GB of RAM.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>