cmd: Fixup DT to pass PStore Ramoops parameters

To simplify configuration and keep synchronized the PStore/Ramoops between
U-Boot and the Linux kernel, this commit dynamically adds the Ramoops
parameters defined in the U-Boot session to the Device Tree.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>