[POWERPC] Raise the upper limit of NR_CPUS and move the pacas into the BSS

This adds the required functionality to fill in all pacas at runtime.

With NR_CPUS=1024
text    data     bss     dec     hex filename
 137 1704032       0 1704169  1a00e9 arch/powerpc/kernel/paca.o :Before
 121 1179744  524288 1704153  1a00d9 arch/powerpc/kernel/paca.o :After

Also remove unneeded #includes from arch/powerpc/kernel/paca.c

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>