ARM: dts: dra7: fix cpsw mdio fck clock

The DRA7 CPSW MDIO functional clock (gmac_clkctrl DRA7_GMAC_GMAC_CLKCTRL 0)
is specified incorrectly, which is caused incorrect MDIO bus clock
configuration MDCLK. The correct CPSW MDIO functional clock is
gmac_main_clk (125MHz), which is the same as CPSW fck. Hence fix it.

Fixes: 1faa415c ("ARM: dts: Add fck for cpsw mdio for omap variants")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>