relocating fixed-clock entry, based on upstream advice

See upstream -
raspberrypi/documentation#1671
raspberrypi/documentation#1673

Quoted here:

* Only Device Tree nodes at the top level of the tree and children of
  a bus node will be probed. For nodes added at run-time there is the
  further limitation that the bus must register for notifications of
  the addition and removal of children. However, there are exceptions
  that break this rule and cause confusion: the kernel explicitly
  scans the entire tree for some device types - clocks and interrupt
  controller being the two main ones - in order to (for clocks)
  initialise them early and/or (for interrupt controllers) in a
  particular order. This search mechanism only happens during booting
  and so doesn't work for nodes added by an overlay at run-time. It is
  therefore recommended for overlays to place fixed-clock nodes in the
  root of the tree unless it is guaranteed that the overlay will not
  be used at run-time.

seeed-4mic-voicecard-overlay.dts

@@ -13,7 +13,7 @@
 	};
 
 	fragment@1 {
-		target-path = "/clocks";
+		target-path = "/";
 		__overlay__ {
 			ac108_mclk: codec-mclk {
 				compatible = "fixed-clock";

seeed-8mic-voicecard-overlay.dts

@@ -13,7 +13,7 @@
 	};
 
 	fragment@1 {
-		target-path = "/clocks";
+		target-path = "/";
 		__overlay__ {
 			ac10x_mclk: codec-mclk {
 				compatible = "fixed-clock";