[POWERPC] Abolish unused ucBoardRev variables

asm-powerpc/processor.h declares, and arch/ppc/platforms/prep_setup.c
defines variables ucBoardRev, ucBoardRevMaj and ucBoardRevMin which
are used nowhere in the current kernel (neither in arch/ppc nor
arch/powerpc).  This removes them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>