PCI: dwc: Refactor core initialization code for EP mode

Split core initialization code for EP mode into two, one that doesn't
touch core registers and the other that touches core registers. The latter
would be called/skipped based on the EPC feature 'core_init_notifier'.

In platforms where this is skipped, it would be called indirectly
through hooks from the endpoint function driver.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>