x86: minnowmax: Add a environment variable for USB power-on delay

x86: minnowmax: Add a environment variable for USB power-on delay

Occasionally it was observed that on Intel MinnowMax board, with a
USB 2.0 device connected to the bottom port, when doing 'usb start'
on the xHCI controller:

  scanning bus 0 for devices... cannot reset port 3!?

But neither of the two USB ports is routed to xHCI root port 3.
Adding some debug information shows that xHCI port 3 PORTSC register
mysteriously reports both CCS = 1 and CSC = 1.

This is not seen every time. After increasing the timeout to wait
for power to become stable, the issue is gone. So this indicates
current default USB power-on delay (20ms) might be at a critical
region where power is stable/unstable. U-Boot provides a mechanism
to have a environment variable to override the default one. Add
one for MinnowMax.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>