Hi,
I do not know where the api function GUI_ALLOC_AssignMemory(); is? is it included in SDK package for LPC?
Anyway, as a workaround, I suppose you can declare a data pointer, for example
uint64_t *MemoryBuff;
MemoryBuff=(uint64_t)0xA0000_0000; //dependent the SDRAM chip select pin
Note that the low three bits 2:0 must be zero, so the pointer must be uint64_t type.

Hope it can help you
BR
XiangJun Rong