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: https://github.com/raspberrypi/linux/issues/2924