usb: ehci-orion: add optional PHY support

This commit extends the ehci-orion so that it can optionally be passed
a reference to a PHY through the Device Tree. It will be useful for
the Armada 375 SoCs. If no PHY is provided then the behavior of the
driver is unchanged.

[Thomas: use devm_phy_optional_get() so that we handle -EPROBE_DEFER
properly. Also call phy_power_off() when needed, and rename goto
labels.]

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>