pico-imx7d: Add SPL support

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>