powerpc/Makefile: Drop CONFIG_WORD_SIZE for BITS

Commit 2578bfae ("[POWERPC] Create and use CONFIG_WORD_SIZE") added
CONFIG_WORD_SIZE, and suggests that other arches were going to do
likewise.

But that never happened, powerpc is the only architecture which uses it.

So switch to using a simple make variable, BITS, like x86, sh, sparc and
tile. It is also easier to spell and simpler, avoiding any confusion
about whether it's defined due to ordering of make vs kconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>