Now that the 'phy-mode' property can be a string array containing more
PHY modes (all that are supported by the board), update the bitmap of
interfaces supported by the MAC with this property.

Normally this would be a simple intersection (of interfaces supported by
the current implementation of the driver and interfaces supported by the
board), but we need to keep being backwards compatible with older DTs,
which may only define one mode, since, as Russell King says,
  conventionally phy-mode has meant "this is the mode we want to operate
  the PHY interface in" which was fine when PHYs didn't change their
  mode depending on the media speed

An example is DT defining
  phy-mode = "sgmii";
but the board supporting also 1000base-x and 2500base-x.

Add the following logic to keep this backwards compatiblity:
- if more PHY modes are defined, do a simple intersection
- if one PHY mode is defined:
  - if it is sgmii, 1000base-x or 2500base-x, add all three and then do
    the intersection
  - if it is 10gbase-r or usxgmii, add both, and also 5gbase-r,
    2500base-x, 1000base-x and sgmii, and then do the intersection

This is simple enough and should work for all boards.

Nonetheless it is possible (although extremely unlikely, in my opinion)
that a board will be found that (for example) defines
  phy-mode = "sgmii";
and the MAC drivers supports sgmii, 1000base-x and 2500base-x, but the
board DOESN'T support 2500base-x, because of electrical reasons (since
the frequency is 2.5x of sgmii).
Our code will in this case incorrectly infer also support for
2500base-x. To avoid this, the board maintainer should either change DTS
to
  phy-mode = "sgmii", "1000base-x";
and update device tree on all boards, or, if that is impossible, add a
fix into the function we are introducing in this commit.

Signed-off-by: Marek Behún <kabel@kernel.org>
---
 drivers/net/phy/phylink.c | 63 +++++++++++++++++++++++++++++++++++++++
 include/linux/phy.h       |  6 ++++
 2 files changed, 69 insertions(+)

diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
index f7156b6868e7..6d7c216a5dea 100644
--- a/drivers/net/phy/phylink.c
+++ b/drivers/net/phy/phylink.c

@@ -563,6 +563,67 @@ static int phylink_parse_fixedlink(struct phylink *pl,
 	return 0;
 }
 
+static void phylink_update_phy_modes(struct phylink *pl,
+				     struct fwnode_handle *fwnode)
+{
+	unsigned long *supported = pl->config->supported_interfaces;
+	DECLARE_PHY_INTERFACE_MASK(modes);
+
+	if (fwnode_get_phy_modes(fwnode, modes) < 0)
+		return;
+
+	if (phy_interface_empty(modes))
+		return;
+
+	/* If supported is empty, just copy modes defined in fwnode. */
+	if (phy_interface_empty(supported))
+		return phy_interface_copy(supported, modes);