PCI: dwc: dra7xx: Refactor Kconfig and Makefile handling for host/ep mode

Refactor the Kconfig and Makefile handling for host/ep mode, since
the previous handling was a bit unorthodox and would have been a bit
bloated once more DWC based controllers added support for ep mode.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>