core: kernel_generic_entry_a64.S: support CFG_DT_ADDR

Add CFG_DT_ADDR for a64 to override the DT address passing
through arg2.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>

core/arch/arm/kernel/generic_entry_a64.S

@@ -54,7 +54,11 @@
 
 FUNC _start , :
 	mov	x19, x0		/* Save pagable part address */
+#if defined(CFG_DT_ADDR)
+	ldr     x20, =CFG_DT_ADDR
+#else
 	mov	x20, x2		/* Save DT address */
+#endif
 
 	adr	x0, reset_vect_table
 	msr	vbar_el1, x0