This adds USB host support to the Meson GXL SoC. A dwc3 controller is
used for host-mode, while a dwc2 controller (not added in this patch
because I could not get it working) is used for device-mode only.

The dwc3 controller's internal roothub has two USB2 ports enabled but no
USB3 port. Each of the ports is supplied by a separate PHY. The USB pins
are connected to the SoC's USBHOST_A and USBOTG_B pins.
Due to the way the roothub works internally the USB PHYs are left
enabled. When the dwc3 controller is disabled the PHY is never powered on
so it does not draw any extra power. However, when the dwc3 host
controller is enabled then all PHYs also have to be enabled, otherwise
USB devices will not be detected (regardless of whether they are plugged
into an enabled port or not). This means that only the dwc3 controller
has to be enabled on boards with USB support (instead of requiring all
boards to enable the PHYs additionally with the chance of forgetting to
enable one and breaking all other ports with that as well).

This also adds the USB3 PHY which currently only does some basic
initialization. That however is required because without it high-speed
devices (like USB thumb drives) do not work on some devices (probably
because the bootloader does not configure the USB3 PHY registers).
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>