ddr: marvell: a38x: Perform DDR training sequence again for 2nd boot

- DDR Training sequence happens very fast. The speedup in boot time is
negligible by skipping the training sequence during 2nd boot or after.
So remove the check and skip.
- This change improves the robustness of DDR training. If u-boot crashed
during DDR training, the training could be left in a limbo state, where
the BootROM has recorded that it is already in a 2nd boot. The training
must be repeated in this scenario to get out of this limbo state, but due
to the check it cannot be performed.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>