arm64: dts: imx8m: correct assigned clocks for FEC

commit 70eacf42

 upstream.

CLK_ENET_TIMER assigned clocks twice, should be a typo, correct to
CLK_ENET_PHY_REF clock.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>