Mainline U-Boot with SPI, NVMe and SATA boot support

[sigmaris]

On the topic of USB boot, I've managed to get a USB-3 Samsung M3 Portable hard drive and done some testing.

With this mainline U-Boot build:
- USB 2.0 flash drive SanDisk Cruzer Blade

• plugged into USB-2 port: it's recognised by U-Boot and can be used
  
• plugged into USB-3 port: it's recognised by U-Boot and can be used
  
• plugged into USB-C port with adapter: it's not detected
  

- USB 3.0 HDD Samsung M3 Portable:

• plugged into USB-2 port: it's recognised by U-Boot and can be used
  
• plugged into USB-3 port: it's not detected
  
• plugged into USB-C port with adapter: it's not detected
  

With U-Boot 2017.09-rockchip-ayufan-1065-g95f6152134:
- USB 2.0 flash drive SanDisk Cruzer Blade

• plugged into USB-2 port: it's recognised by U-Boot
  
• plugged into USB-3 port: it's recognised by U-Boot
  
• plugged into USB-C port with adapter: it's not detected
  

- USB 3.0 HDD Samsung M3 Portable:

• plugged into USB-2 port: it's not detected
  
• plugged into USB-3 port: it's not detected
  
• plugged into USB-C port: it's not detected
  

I'd guess in mainline U-Boot there is support for USB 2.0, but not full USB 3.0 or USB Type-C support. This is backed up by this comment on the original submission of the USB support from Rockchip staff: https://lists.denx.de/pipermail/u-boot/2...64557.html

Only support usb2.0 currently for we have not enable the usb3.0 phy driver and PD(fusb302) driver.

So USB 3.0 devices will probably only work if they are forced to work in USB 2.0 mode, e.g. by being plugged into the USB 2.0 ports.