This patch changes the USB port scanning procedure and timeout
handling in the following ways:

a)
The power-on delay in usb_hub_power_on() is now reduced to a value of
max(100ms, "hub->desc.bPwrOn2PwrGood * 2"). The code does not wait
using mdelay, instead usb_hub_power_on() will wait before querying
the device in the scanning loop later. The total timeout for this
hub, which is 1 second + "hub->desc.bPwrOn2PwrGood * 2" is calculated
and will be used in the following per-port scanning loop as the timeout
to detect active USB devices on this hub.

b)
Don't delay the minimum delay (for power to stabilize) in
usb_hub_power_on(). Instead skip querying these devices in the scannig
loop until the delay time is reached.

c)
The ports are now scanned in a quasi parallel way. The current code did
wait for each (unconnected) port to reach its timeout and only then
continue with the next port. This patch now changes this to scan all
ports of all USB hubs quasi simultaneously. For this, all ports are added
to a scanning list. This list is scanned until all ports are ready
by either a) reaching the connection timeout (calculated earlier), or
by b) detecting a USB device. This results in a faster USB scan time as
the recursive scanning of USB hubs connected to the hub that's currently
being scanned will start earlier.

One small functional change to the original code is, that ports with
overcurrent detection will now get rescanned multiple times
(PORT_OVERCURRENT_MAX_SCAN_COUNT).

Without this patch:
starting USB...
USB0:   USB EHCI 1.00
scanning bus 0 for devices... 9 USB Device(s) found

time: 20.163 seconds

With this patch:
starting USB...
USB0:   USB EHCI 1.00
scanning bus 0 for devices... 9 USB Device(s) found

time: 1.822 seconds

So ~18.3 seconds of USB scanning time reduction.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Stephen Warren <swarren@nvidia.com>