Add subdirectories to engine/ similar to pokecrystal

engine/gfx/oam_dma.asm

@@ -0,0 +1,26 @@
+WriteDMACodeToHRAM::
+; Since no other memory is available during OAM DMA,
+; DMARoutine is copied to HRAM and executed there.
+	ld c, $ff80 % $100
+	ld b, DMARoutineEnd - DMARoutine
+	ld hl, DMARoutine
+.copy
+	ld a, [hli]
+	ld [$ff00+c], a
+	inc c
+	dec b
+	jr nz, .copy
+	ret
+
+DMARoutine:
+	; initiate DMA
+	ld a, wOAMBuffer / $100
+	ld [rDMA], a
+
+	; wait for DMA to finish
+	ld a, $28
+.wait
+	dec a
+	jr nz, .wait
+	ret
+DMARoutineEnd: