Support LAN8720 and DP83848 PHY models

[wdx04]

Summary of changes

Fix #454 .
Support a range of models in a single Ethernet PHY driver.
Renamed LAN8742 driver LAN87XX to support both LAN8742 and LAN8720 in a single driver.
Added DP83848 PHY driver.
Added missing GPIO_InitStructure declarations in the EthInitPinmappings function for some STM32 boards.

Impact of changes

Migration actions required

If an application explicitly specified the EMAC PHY model with "nsapi.emac-phy-model": "LAN8742" in mbed_app.json5, it needs to be renamed LAN87XX.

Documentation

To use the LAN8720 or LAN8742 PHY, override the nsapi.emac-phy-model setting in mbed_app.json5, or just remove the nsapi.emac-phy-model and nsapi.emac-phy-mdio-address entries to use the default value:

"MY_TARGET": {
    "nsapi.emac-phy-model": "LAN87XX",
    "nsapi.emac-phy-mdio-address": 0
}

To use the DP83848 PHY, override the nsapi.emac-phy-model setting in mbed_app.json5:

"MY_TARGET": {
    "nsapi.emac-phy-model": "DP8384X",
    "nsapi.emac-phy-mdio-address": 1
}

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Tested with a simple TCP Ehco application on three boards:

STM32F746G-DISCOVERY with LAN8742
Custom STM32F407 board with LAN8720
Custom STM32F407 board with DP83848

The application running on the board acts as a TCP client. It connects to the server running on the PC, and sends some text to the server. The server sends back the same text after receiving it, and the client verifies whether it is the same as the sent text.

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[multiplemonomials]

Thanks a ton for the fix!