board/b4860qds:Slow MDC clock to comply IEEE specs in PBI config

The MDC generate by default value of MDIO_CLK_DIV is too high i.e. higher
than 2.5 MHZ.  It violates the IEEE specs.

So Slow MDC clock to comply IEEE specs

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>