arm: dts: Add coherent_pool=1M to Pi 4 bootargs

Downstream Raspberry Pi dts files add "coherent_pool=1M" to the kernel
command line to aid the dwc_otg driver, but this excluded Pi 4 which
uses a new XCHI interface instead. UAS also benefits from a larger
coherent_pool value, so replicate the addition in bcm2711-rpi-4-b.dts.

See: https://github.com/raspberrypi/linux/pull/3040



Signed-off-by: Phil Elwell <phil@raspberrypi.org>