net: ethernet: stmmac: simplify phy modes management for stm32

No new feature, just to simplify stm32 part to be easier to use.
Add by default all Ethernet clocks in DT, and activate or not in function
of phy mode, clock frequency, if property "st,ext-phyclk" is set or not.
Keep backward compatibility
--------------------------------------------------------------------------
|PHY_MODE |    Normal | PHY wo crystal|   PHY wo crystal   |  No 125Mhz  |
|         |	      |      25MHz    |        50MHz       |  from PHY   |
--------------------------------------------------------------------------
|  MII    |	 -    |     eth-ck    |       n/a          |	    n/a  |
|         |	      | st,ext-phyclk |                    |             |
--------------------------------------------------------------------------
|  GMII   |	 -    |     eth-ck    |       n/a          |	    n/a  |
|         |	      | st,ext-phyclk |                    |             |
--------------------------------------------------------------------------
| RGMII   |	 -    |     eth-ck    |       n/a          |     eth-ck  |
|         |	     | st,ext-phyclk |                    |st,eth-clk-sel|
|         |	     |               |                    |       or     |
|         |	     |               |                    | st,ext-phyclk|
----------------==--------------------------------------------------------
| RMII    |	 -   |     eth-ck    |      eth-ck        |       n/a    |
|         | 	     | st,ext-phyclk | st,eth-ref-clk-sel |              |
|         | 	     |               | or st,ext-phyclk   |              |
--------------------------------------------------------------------------

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>