MIPS: BCM63XX: select BMIPS4350 and default to 2 CPUs for supported SoCs

All BCM63XX SoCs starting with BCM6358 have a BMIPS4350 instead of a
BMIPS3300, so select it unless support for any of the older SoCs is
selected.
All BMIPS4350 have only two CPUs, so select the appropriate default.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/5355/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>