stmmac: MDC clock dynamically based on the csr clock input

If a specific clk_csr value is passed from the platform
this means that the CSR Clock Range selection cannot be
changed at run-time and it is fixed (as reported in the driver
documentation). Viceversa the driver will try to set the MDC
clock dynamically according to the actual clock input.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Francesco Virlinzi <francesco.virlinzi@st.com>
Reviewed-by: David Laight <david.laight@aculab.com>
Signed-off-by: David S. Miller <davem@davemloft.net>