DTS: overlays: add mmio-hi parameter to pciex1-compat-pi5

I225-V network adapters have a buggy ROM that won't complete internal
initialisation if (at least) BAR0 has an assigned address of 0x0.

Add a parameter to the pciex1-compat-pi5 overlay to make outbound
addresses start at 2GB.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>