ddr: Add DDR driver for iMX8ULP
    
    Add iMX8ULP DDR initialization driver which loads the DDR timing
    parameters and executes the training procedure.
    
    When enabling IMX8ULP_DRAM_PHY_PLL_BYPASS, using PHY PLL bypass mode
    to do DDR init
    
    Signed-off-by: Ye Li <ye.li@nxp.com>