[POWERPC] 32-bit early_init() should zero from __bss_start to __bss_stop only

Currently, early_init() in setup_32.c zeroes from '_bss_start' to '_end'.
It should only zero from '__bss_start' to '__bss_stop'.  This patch does that.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>