stmmac: Define MDC clock selection macros

The patch adds the macros to be used for MDC clock selection. The MDC clock
frequency is based on scaled system clock, and has to be confined to a range
of 1-2.5 MHz. Based on the input CSR clock, the scaling factor has to be
selected.
The platform specific code will provide the default value of this scaling
factor, based on the input CSR clock.
There is an option to set MDC clock higher than the IEEE 802.3 specified
frequency limit of 2.5 MHz. This applies for the interfacing chips that
support higher MDC clocks. The resultant higher clock of 12.5 MHz requires
additional Macros to be defined for the clock divider corresponding to the
to the following selection.
-----------------------------------------
	Selection	MDC Clock
-----------------------------------------
	1000 		clk_csr_i/4
	1001 		clk_csr_i/6
	1010 		clk_csr_i/8
	1011 		clk_csr_i/10
	1100 		clk_csr_i/12
	1101	 	clk_csr_i/14
	1110 		clk_csr_i/16
	1111 		clk_csr_i/18

This support has to be added both in the include file, as well as driver. The
driver need to program the registers based on the interfacing chips. This would
be more board specific information and needs to be passed through the platform
code to the driver. This work would be carried out in the future patch set
release.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>