powerpc/configs: Add CONFIG_NR_CPUS to ppc64_defconfig

CONFIG_NR_CPUS is not set in ppc64_defconfig, So it gets default of 32
which is quite small for modern powerpc systems. Instead set a default
of 2048 like other powerpc defconfigs.

Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>