PCI: dra7xx: Add TI DRA7xx PCIe driver

Add support for PCIe controller in DRA7xx.  This driver re-uses the
designware core code that is already present in kernel.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Cc: Mohit Kumar <mohit.kumar@st.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Arnd Bergmann <arnd@arndb.de>