pci: dwc: pci-dra7xx: Make shutdown handler static

Define dra7xx_pcie_shutdown() as a static function as it is not used
in other compilation units.

Signed-off-by: Vignesh R <vigneshr@ti.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Keerthy <j-keerthy@ti.com>