hikey960: Set alignment size 512B for fip building

Set alignment size to 512B so finally we can get fip.bin with 512B
alignment. This can avoid stuck issue for 'fastboot' downloading
if USB driver uses DMA for data transferring.
Signed-off-by: Leo Yan <leo.yan@linaro.org>

plat/hisilicon/hikey960/platform.mk

@@ -100,3 +100,5 @@ BL31_SOURCES		+=	drivers/arm/cci/cci.c			\
 ERRATA_A53_836870		:=	1
 ERRATA_A53_843419		:=	1
 ERRATA_A53_855873		:=	1
+
+FIP_ALIGN			:=	512