Documentation: Document the RB532 specific kmac tag

The Routerboard 532 bootloader passes the korina ethernet
MAC adapter address to the kernel on the command line.
Document this in the kernel-parameters file.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>