ram: rk3399: Conduct memory training at 400MHz

ram: rk3399: Conduct memory training at 400MHz

Currently the default initialisation frequency is 50MHz.  Although
this does appear to be suitable for some LPDDR4 RAM chips, training at
this low frequency has been seen to cause Column errors, leading to
Capacity check errors on others.

Here we force RAM initialisation to happen at 400MHz before ramping up
to the final value running value of 800MHz after everything has been
successfully configured.

Link: https://lore.kernel.org/u-boot/Yo4v3jUeHXTovjOH@google.com/
Suggested-by: YouMin Chen <cym@rock-chips.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Tested-by: Xavier Drudis Ferran <xdrudis@tinet.cat>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Michal Suchánek <msuchanek@suse.de>