ram: rk3399: Add set_rate sdram rk3399 ops

ram: rk3399: Add set_rate sdram rk3399 ops

DDR set rate can be even required for lpddr4 and we
need to keep the lpddr4 code to compile only for relevant
boards which do support lpddr4.

For this requirement, and for code readability handle
data training via sdram_rk3399_ops with .set_rate and
same will update in future while supporting lpddr4 code.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>