MIPS: BCM47XX: only print SoC name in system type in cpuinfo

Recently the output of "system type" in  /proc/cpuinfo was changed to
Broadcom BCM4730 (Some sample board), but it is better to just print
the SoC name in the "system type" entry. The board name will be added
in the machine entry later.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5865/