ddr: imx: Save the FW loading if it hasn't changed

Function ddr_load_train_firmware() is called 4 times in a loop by
ddr_cfg_phy(). The first 3 calls are all '1D' type and just loading the
same FWs.  Let's add a type check and save 2 of them.  This helps to
reduce DDRPHY training time from 269 ms down to 212 ms, and thus speed
up boot time ~ 50 ms.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Fabio Estevam <festevam@denx.de>