kasan: Document support on 32-bit powerpc

KASAN is supported on 32-bit powerpc and the docs should reflect this.

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YoTEnMLrnd64j0w5@cleo