Add a helper to allow ethernet drivers to limit the speed of a phy
(that they are attached to).

This mainly involves factoring out the business-end of
of_set_phy_supported() and exporting a new symbol.

This code seems to be open coded in several places, in several different
variants.

This code is envisaged this will be used in situations where setting
the "max-speed" property is not appropriate, e.g. because the maximum
speed is not a property of the phy hardware.

Signed-off-by: Simon Horman <redacted>

---

v2
* First post
---
  drivers/net/phy/phy_device.c | 52 ++++++++++++++++++++++++++++----------------
  include/linux/phy.h          |  1 +
  2 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
index c0f211127274..d9a020095972 100644
--- a/drivers/net/phy/phy_device.c
+++ b/drivers/net/phy/phy_device.c

@@ -1205,6 +1205,37 @@ static int gen10g_resume(struct phy_device *phydev)
  	return 0;
  }

+static void __set_phy_supported(struct phy_device *phydev, u32 max_speed)
+{
+	if (!IS_ENABLED(CONFIG_OF_MDIO))
+		return;
+
+	/* The default values for phydev->supported are provided by the PHY
+	 * driver "features" member, we want to reset to sane defaults fist