ARM: bcm283x: change the virtual address of the XHCI PCI device base

Move the XHCI PCI device base up in the virtual address space. This fixes
initialization failure observed with newer Raspberry Pi firmware, later
than 63b1922311 ("firmware: arm_loader: Update armstubs with those from
PR 117). It looks that chosing 0xff800000 as the XHCI PCI device base
conflicts with the updated ARM/VideoCore firmware.

This also requires to reduce the size of the mapped PCI device region
from 8MiB to 4MiB to fit into 32bit address space. This is still enough
for the XHCI PCI device.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Tested-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>