rockchip: rename board-spl.c to rk3288-board-spl.c

since different rockchip soc need different spl file,
so rename board-spl.c.

Signed-off-by: Lin Huang <hl@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>

arch/arm/mach-rockchip/Makefile

@@ -5,7 +5,7 @@
 #
 
 ifdef CONFIG_SPL_BUILD
-obj-y += board-spl.o
+obj-$(CONFIG_ROCKCHIP_RK3288) += rk3288-board-spl.o
 else
 obj-y += board.o
 endif