rockchip: Allow jerry to use of-platdata

This board always boots from SPI, so update the code to support that with
of-platdata. The boot source is not currently available with of-platdata.
Signed-off-by: NSimon Glass <sjg@chromium.org>

arch/arm/mach-rockchip/rk3288-board-spl.c

@@ -64,6 +64,8 @@ u32 spl_boot_device(void)
 	}
 
 fallback:
+#elif defined(CONFIG_TARGET_CHROMEBOOK_JERRY)
+	return BOOT_DEVICE_SPI;
 #endif
 	return BOOT_DEVICE_MMC1;
 }