ARM: dts: omap3: Fix am3517 mdio and emac clock references

A previous patch removed OMAP clock aliases that were perceived
to be unnecessary.  Unfortunately, it broke the ethernet on the
am3517-evm.  This patch enables the MDIO clock and EMAC clock.

Fixes: 0ed266d7

 ("clk: ti: omap3: cleanup unnecessary clock aliases")
Cc: stable@vger.kernel.org #4.16+

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>