ARM: dts: socfpga: Fix the ethernet clock phandle

The ethernet block clock phandle must point to the clock node which
represents the clock which directly supply the ethernet block. This
is emac_x_clk , not emacx_clk , so fix this.

From: Pavel Machek <pavel@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>