ARM: davinci: da850: don't add emac clock to lookup table twice

Similarly to the aemif clock - this screws up the linked list of clock
children. Create a separate clock for mdio inheriting the rate from
emac_clk.

Cc: <stable@vger.kernel.org> # 3.12.x-
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[nsekhar@ti.com: add a comment over mdio_clk to explaing its existence +
		 commit headline updates]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>