realtek: backport Clause-45 MDIO helper functions

Import commit ("c6af53f038aa3 net: mdio: add helpers to extract clause
45 regad and devad fields") from Linux 5.17 to allow making the MDIO
code in the ethernet driver more readable.

Signed-off-by: Daniel Golle <[email protected]>

target/linux/realtek/patches-5.10/020-v5.17-net-mdio-add-helpers-to-extract-clause-45-regad-and-.patch

@@ -0,0 +1,53 @@
+From c6af53f038aa32cec12e8a305ba07c7ef168f1b0 Mon Sep 17 00:00:00 2001
+From: "Russell King (Oracle)" <[email protected]>
+Date: Tue, 4 Jan 2022 12:07:00 +0000
+Subject: [PATCH 2/3] net: mdio: add helpers to extract clause 45 regad and
+ devad fields
+
+Add a couple of helpers and definitions to extract the clause 45 regad
+and devad fields from the regnum passed into MDIO drivers.
+
+Tested-by: Daniel Golle <[email protected]>
+Reviewed-by: Andrew Lunn <[email protected]>
+Signed-off-by: Russell King (Oracle) <[email protected]>
+Signed-off-by: Daniel Golle <[email protected]>
+Signed-off-by: David S. Miller <[email protected]>
+---
+ include/linux/mdio.h | 12 ++++++++++++
+ 1 file changed, 12 insertions(+)
+
+--- a/include/linux/mdio.h
++++ b/include/linux/mdio.h
+@@ -7,6 +7,7 @@
+ #define __LINUX_MDIO_H__
+ 
+ #include <uapi/linux/mdio.h>
++#include <linux/bitfield.h>
+ #include <linux/mod_devicetable.h>
+ 
+ /* Or MII_ADDR_C45 into regnum for read/write on mii_bus to enable the 21 bit
+@@ -14,6 +15,7 @@
+  */
+ #define MII_ADDR_C45		(1<<30)
+ #define MII_DEVADDR_C45_SHIFT	16
++#define MII_DEVADDR_C45_MASK	GENMASK(20, 16)
+ #define MII_REGADDR_C45_MASK	GENMASK(15, 0)
+ 
+ struct gpio_desc;
+@@ -342,6 +344,16 @@ static inline u32 mdiobus_c45_addr(int d
+ 	return MII_ADDR_C45 | devad << MII_DEVADDR_C45_SHIFT | regnum;
+ }
+ 
++static inline u16 mdiobus_c45_regad(u32 regnum)
++{
++	return FIELD_GET(MII_REGADDR_C45_MASK, regnum);
++}
++
++static inline u16 mdiobus_c45_devad(u32 regnum)
++{
++	return FIELD_GET(MII_DEVADDR_C45_MASK, regnum);
++}
++
+ static inline int __mdiobus_c45_read(struct mii_bus *bus, int prtad, int devad,
+ 				     u16 regnum)
+ {