PCI: dra7xx: Get an optional clock

If the clock is provided externally we need to make sure it is enabled
before starting PCI scan.

Link: https://lore.kernel.org/r/20210531085934.2662457-5-luca@lucaceresoli.net


Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>