net: add MDIO_MUX DM class

Adds a class for MDIO MUXes, which control access to a series of
downstream child MDIOs.
MDIO MUX drivers are required to implement a select function used to switch
between child buses.
MUX children are registered as MDIO buses and they can be used just like
regular MDIOs.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

include/dm/uclass-id.h

@@ -59,6 +59,7 @@ enum uclass_id {
 	UCLASS_MAILBOX,		/* Mailbox controller */
 	UCLASS_MASS_STORAGE,	/* Mass storage device */
 	UCLASS_MDIO,		/* MDIO bus */
+	UCLASS_MDIO_MUX,	/* MDIO MUX/switch */
 	UCLASS_MISC,		/* Miscellaneous device */
 	UCLASS_MMC,		/* SD / MMC card or chip */
 	UCLASS_MOD_EXP,		/* RSA Mod Exp device */