Add support for boot arguments specification via fdt

Add suppport for passing boot arguments via FDT for mediatek/ralink SoCs.
This was taken from kan's work on CI20.

Since most OpenWRT dts files have bootargs defined, we use bsdbootargs
to specify FreeBSD specific arguments.

Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5979