[mips/broadcom] set sysctl "hw.model" by SoC ChipCommon ID

Small patch to set "hw.model" by SoC ChipCommon ID. On BCM4718 it will be BCM4716, because 4716 is core for 4716,4717 & 4718.

Tested on Netgear WNR3500L (BCM4718) and Asus RT-N53 (BCM5357).

Reviewed by: landonf, yamori813@yahoo.co.jp
Sponsored by: Postgres Professional
Differential revision: https://reviews.freebsd.org/D12737