dtoverlay: Reduce size of PCIE IB window in pcie-32-dma overlay

The PCIE inbound window is rounded up to a power of 2, so the default
of 3GB rounds up to 4GB starting at 0. This prohibits the MSI vector
sitting at 0x0_fffffffc, and causes warnings from some subsystems
(eg ahci) of a 64bit address on a 32bit configuration.

Reduce the window down to 2GB to avoid this issue.

https://github.com/raspberrypi/linux/issues/4848



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>