powerpc/kgdb: Do not set kgdb_single_step on ppc

The kgdb_single_step flag has the possibility to indefinitely
hang the system on an SMP system.

The x86 arch have the same problem, and that problem was fixed by
commit 8097551d

(kgdb,x86: do not set kgdb_single_step
on x86). This patch does the same behaviors as x86's patch.

Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>