The RK3399 is capable of driving DDR3 at 933MHz (i.e. DDR3-1866),
if the PCB layout permits and appropriate memory timings are used.

This changes the sanity checks to allow a DTS to request DDR3-1866
operation.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>