rockchip: Use of-platdata for firefly-rk3288

As an experiment, move this board over to use of-platdata. This means that
its SPL configuration will come from C structures generated at build-time
from the device tree, instead of coming from the device tree at run-time.

Signed-off-by: Simon Glass <sjg@chromium.org>

configs/firefly-rk3288_defconfig

@@ -69,3 +69,6 @@ CONFIG_USE_PRIVATE_LIBGCC=y
 CONFIG_USE_TINY_PRINTF=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_ERRNO_STR=y
+CONFIG_SPL_OF_PLATDATA=y
+# CONFIG_SPL_OF_LIBFDT is not set
+CONFIG_ROCKCHIP_SERIAL=y