powerpc: fix build when CONFIG_BOOKE_WDT is enabled

Commit ae3a197e

 (Disintegrate asm/system.h for PowerPC) broke build of
assembly files when CONFIG_BOOKE_WDT is enabled as follows:

  AS      arch/powerpc/lib/string.o
/home/baruch/git/stable/arch/powerpc/include/asm/reg_booke.h: Assembler messages:
/home/baruch/git/stable/arch/powerpc/include/asm/reg_booke.h:19: Error: Unrecognized opcode: `extern'
/home/baruch/git/stable/arch/powerpc/include/asm/reg_booke.h:20: Error: Unrecognized opcode: `extern'

Since setup_32.c is the only user of the booke_wdt configuration variables, move
the declarations there.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>