dts: Increase default coherent pool size

dwc_otg allocates DMA-coherent buffers in atomic context for misaligned
transfer buffers. The pool that these allocations come from is set up
at boot-time but can be overridden by a commandline parameter -
increase this for now to prevent failures seen on 4.19 with multiple
USB Ethernet devices.

see: #2924

arch/arm/boot/dts/bcm270x.dtsi

@@ -3,7 +3,7 @@
 
 / {
 	chosen {
-		bootargs = "";
+		bootargs = "coherent_pool=1M";
 		/delete-property/ stdout-path;
 	};