MIPS: generic/yamon-dt: Pull YAMON DT shim code out of SEAD-3 board

MIPS: generic/yamon-dt: Pull YAMON DT shim code out of SEAD-3 board

In preparation for supporting other YAMON-using boards (Malta) & sharing
code to translate information from YAMON into device tree properties,
pull the code doing so for the kernel command line, system memory &
serial configuration out of the SEAD-3 board code.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16181/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>