armv8/ls2085a_emu: Drop DDR3 emulation target

The emulator with DDR3 model was used during model bringup. DDR4
controllers are used with ls2085a. Drop the DDR4 target defconfig
and enable DDR4 in ls2085a_emu_defconfig.

Signed-off-by: York Sun <yorksun@freescale.com>