pico-imx7d: Add SPL support

Convert pico-imx7d to SPL support.

There are two variants of pico-imx7d SOMs:
- One with 512MB of RAM
- One with 1GB of RAM

The 512MB module contains two Hynix H5TC2G63GFR-PBA.
The 1GB module contains two Hynix H5TC4G63GFR-PBA.

The RAM size is determined in runtime by reading GPIO1_12.

While at it, also add USB Serial Download mode support as it
is very helpful for loading SPL and u-boot.img via imx_usb_loader.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
diff --git a/board/technexion/pico-imx7d/Makefile b/board/technexion/pico-imx7d/Makefile
index 6cfa803..4ae3d60 100644
--- a/board/technexion/pico-imx7d/Makefile
+++ b/board/technexion/pico-imx7d/Makefile
@@ -1,4 +1,4 @@
 # SPDX-License-Identifier: GPL-2.0+
 # (C) Copyright 2017 NXP Semiconductors
 
-obj-y  := pico-imx7d.o
+obj-y  := pico-imx7d.o spl.o