pci_endpoint: pci_cdns_ti_ep: Enable PCIe Endpoint mode in J784S4 SoC

pci_endpoint: pci_cdns_ti_ep: Enable PCIe Endpoint mode in J784S4 SoC

TI's J784S4 SoC has two instances of PCIe Controller namely PCIe0 and
PCIe1 which are Cadence PCIe Controllers. Add support to configure PCIe
instances in Endpoint mode of operation.

While at it disable all endpoint functions except function 0 during
probe to prevent the Root Complex from enumerating unconfigured
functions. This ensures only  properly configured endpoint functions
are visible to the host and avoids enumeration issues with
multi-function devices.

Signed-off-by: Hrushikesh Salunke <h-salunke@ti.com>