[POWERPC] Make PS3_SYS_MANAGER default y, not m

Currently it's impossible to build a ps3_defconfig which will reboot
without modules installed. This makes it all too easy to find yourself
with a PS3 that won't reboot.

This is because the system manager driver, which provides the reboot
mechanism, is only selectable if PS3_ADVANCED is set, else it defaults
to m. In ps3_defconfig PS3_ADVANCED is not set, therefore the system
manager is built as a module.

It would be desirable IMHO for the defconfig to produce a kernel that
boots and reboots, without needing modules to be installed.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>