usb: hub: make minimum power-on delay configurable

This patch makes the minimum power-on delay for USB HUB
become configurable. The original design waits at least
100 msec here, but some EHCI controlers(e.g. Faraday EHCI)
are known to require much longer delay interval.

Signed-off-by: Kuo-Jung Su <dantesu@faraday-tech.com>
CC: Marek Vasut <marex@denx.de>
diff --git a/README b/README
index 33bda8c..ac1ec44 100644
--- a/README
+++ b/README
@@ -1261,6 +1261,9 @@
 		CONFIG_USB_EHCI_TXFIFO_THRESH enables setting of the
 		txfilltuning field in the EHCI controller on reset.
 
+		CONFIG_USB_HUB_MIN_POWER_ON_DELAY defines the minimum
+		interval for usb hub power-on delay.(minimum 100msec)
+
 - USB Device:
 		Define the below if you wish to use the USB console.
 		Once firmware is rebuilt from a serial console issue the