powerpc/defconfigs: Reduce 64bit vmlinux by making acenic and cramfs modules

Machines with acenic adapters are rare these days, so we may as well make it
a module. Cramfs is also very rarely used so we can make it a module.

Together this saves 143kB on a 64bit compile:

   text	   data	    bss	    dec	    hex	filename
8247176	1729404	1221988	11198568	 aae068	vmlinux~
8134997	1727588	1188836	11051421	 a8a19d	vmlinux

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>