On Armada 7K/8K we need to explicitly enable the register clock. This
clock is optional because not all the SoCs using this IP need it but at
least for Armada 7K/8K it is actually mandatory.

The change was done at xhci-plat level and not at a xhci-mvebu.c because,
it is expected that other SoC would have this kind of constraint.

The binding documentation is updating accordingly.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
---
 Documentation/devicetree/bindings/usb/usb-xhci.txt |  5 +++-
 drivers/usb/host/xhci-plat.c                       | 33 ++++++++++++++++++----
 drivers/usb/host/xhci.h                            |  3 +-
 3 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/Documentation/devicetree/bindings/usb/usb-xhci.txt b/Documentation/devicetree/bindings/usb/usb-xhci.txt
index e2ea59bbca93..e4b14511f4f8 100644
--- a/Documentation/devicetree/bindings/usb/usb-xhci.txt
+++ b/Documentation/devicetree/bindings/usb/usb-xhci.txt

@@ -27,7 +27,10 @@ Required properties:
   - interrupts: one XHCI interrupt should be described here.
 
 Optional properties:
-  - clocks: reference to a clock
+  - clocks: reference to the clocks
+  - clock-names: mandatory if there is a second clock, in this case
+    the name must be "core" for the first clock and "reg" for the
+    second one
   - usb2-lpm-disable: indicate if we don't want to enable USB2 HW LPM
   - usb3-lpm-capable: determines if platform is USB3 LPM capable
   - quirk-broken-port-ped: set if the controller has broken port disable mechanism

diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
index 79afaac57ef6..fd0c399013a2 100644
--- a/drivers/usb/host/xhci-plat.c
+++ b/drivers/usb/host/xhci-plat.c

@@ -157,6 +157,7 @@ static int xhci_plat_probe(struct platform_device *pdev)
 	struct resource         *res;
 	struct usb_hcd		*hcd;
 	struct clk              *clk;
+	struct clk              *reg_clk;
 	int			ret;
 	int			irq;
 

@@ -226,17 +227,27 @@ static int xhci_plat_probe(struct platform_device *pdev)
 	hcd->rsrc_len = resource_size(res);
 
 	/*
-	 * Not all platforms have a clk so it is not an error if the
-	 * clock does not exists.
+	 * Not all platforms have clks so it is not an error if the
+	 * clock do not exist.
 	 */
+	reg_clk = devm_clk_get(&pdev->dev, "reg");
+	if (!IS_ERR(reg_clk)) {
+		ret = clk_prepare_enable(reg_clk);
+		if (ret)
+			goto put_hcd;
+	} else if (PTR_ERR(reg_clk) == -EPROBE_DEFER) {
+		ret = -EPROBE_DEFER;
+		goto put_hcd;
+	}
+