net: macb: Set MDIO clock divisor for pclk higher than 160MHz

Currently macb sets clock divisor for pclk up to 160 MHz.
Function gem_mdc_clk_div was updated to enable divisor
for higher values of pclk.

Signed-off-by: Bartosz Wawrzyniak <bwawrzyn@cisco.com>
Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>