Silicon/Bcm27xx: Add PCIe constants to Bcm2711.h

This populates all of the define's we need for PCIe accesses.
Four new PCDs are also introduced for the register and MMIO platform
constants.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>