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>
---
For example:

Board: Xilinx Zynq
Net:   ZYNQ GEM: e000b000, phyaddr 7, interface rgmii-id

Warning: ethernet@e000b000 (eth0) using random MAC address -
7a:fc:90:53:6a:41
eth0: ethernet@e000b000ZYNQ GEM: e000c000, phyaddr ffffffff, interface
rgmii-id

Warning: ethernet@e000c000 (eth3) using random MAC address -
1a:ff:d7:1a:a1:b2
, eth3: ethernet@e000c000
** Bad device size - mmc 0 **
Checking if uenvcmd is set ...
Hit any key to stop autoboot:  0
Zynq> mdio list
eth0:
17 - Marvell 88E1111S <--> ethernet@e000b000
eth3:
17 - Marvell 88E1111S <--> ethernet@e000c000
Zynq>