usb: chipidea: Add support for 'phy-clkgate-delay-us' property

Add support for the optional 'phy-clkgate-delay-us' property that is
used to describe the delay time between putting PHY into low power mode
and turning off the PHY clock.

Signed-off-by: Li Jun <jun.li@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>