Platform/RaspberryPi: fix FDT handling for RPi4

A rev-up of start4.elf VPU firmware meant that the previous scheme of
loading the DTB over top of RPI_EFI.FD no longer works - the DT is now
loaded way before the armstub, so any overlap means the DT is overridden.

This change re-arranges a few items in the FD, allowing the DTB to loaded
directly after the FD in physical memory.

This moves UEFI image down by 0x10000, and reduces the FD image size by
0x10000, leaving space for a DTB to be loaded by config.txt at 0x1f0000.

You need a matching "rev RPi4 TF-A for DTB fix" patch to edk2-non-osi,
as it requires a TF-A build with these options:

PRELOADED_BL33_BASE=0x20000 RPI3_PRELOADED_DTB_BASE=0x1f0000

Note: the same problem still affects the Pi 3, and will be
fixed in a separate change.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>