common: miiphyutil: Add helper function for mdio bus name

The most of ethernet drivers are using this mdio registration sequence.
strcpy(priv->bus->name, "emac");
mdio_register(priv->bus);
Where driver can be used only with one MDIO bus because only unique
name should be used.

Other drivers are using unique device name for MDIO registration to
support multiple instances.
snprintf(priv->bus->name, sizeof(bus->name), "%s", name);

With DM dev->seq is used more even in logs
(like random MAC address generation:
printf("\nWarning: %s (eth%d) using random MAC address - %pM\n",
       dev->name, dev->seq, pdata->enetaddr);
)
where eth%d prefix is used.

Simplify driver code to register mdio device with dev->seq number
to simplify mdio registration and reduce code duplication across
all drivers. With DM_SEQ_ALIAS enabled dev->seq reflects alias setting.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>