Implement support for sysctl hw.model for Mediatek/Ralink SoCs

These SoCs have CHIPID registers, which store the Chip model, according
to the manufacturer; make use of those in order to better identify
the chip we're actually running on.

If we're unable to read the CHIPID registers for some reason we will
use the string "unknown " as a value for hw.model.

Reported by: yamori813@yahoo.co.jp
Sponsored by: Smartcom - Bulgaria AD