Fix DRAM type detection on Dragino 2

Fix DRAM type detection on Dragino 2

Dragino 2 needs custom DRAM type detection as it doesn't follow general
convention. MEM_TYPE value in BOOT_STRAP register isn't set correctly
on this board because only GPIO28 is used for DRAM type detection.

This fixes DRAM type detection on new Dragino 2 version which is based
on Dragino HE module, with DDR2 memory.