config: Enable support for the XHCI controller on RPI4 board

This requires enabling BRCMSTB PCIe and XHCI_PCI drivers as well as PCI
and USB commands. To get it working one has to call the following commands:
"pci enum; usb start;", thus such commands have been added to the default
"preboot" environment variable. One has to update their environment if it
is already configured to get this feature working out of the box.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>