PCI: brcmstb: Add Broadcom STB PCIe host controller driver

This adds a basic driver for Broadcom's STB PCIe controller, for now
aimed at Raspberry Pi 4's SoC, bcm2711.

Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
Co-developed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
[lorenzo.pieralisi@arm.com: updated brcm_pcie_get_rc_bar2_size_and_offset()according to https://lore.kernel.org/linux-pci/be8ddb33a7360af1815cf686f77f3f0913d02be3.camel@suse.de]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Jeremy Linton <jeremy.linton@arm.com>