mt8365: add SPL support and use it as Download Agent (DA)

For Android devices based on mt8365, we will use U-Boot SPL as download agent (DA) when we bootstrap the device in download mode.

The download mode (like recovery mode) is typically used when the device is corrupted or bootloader is failing.

In this mode the goal is to "recover" the device and be able to reboot on a new bootloader.

All the download will be done through USB with DFU.

Flashing process:

$ aiot-bootrom -b da.bin
$ dfu-util -R -w -a 0 -D MBR_EMMC
$ dfu-util -w -a 0 -D bl2.img
$ dfu-util -R -w -a 1 -D fip.bin