PCI: endpoint: Add custom notifier support

Add support for notifying the EPF device about the custom notifications
received by the EPC device from the Root complex. These notifications
are EPC/vendor specific and are shared with the specific EPF devices.

For instance, the Qcom EPC device generates events such as MHI_A7,
MHI_Q6, DEBUG, MMIO_WRITE, CFG_WRITE etc... These events can be passed
to the EPF device using this notifier.

EPC
---

```
static irqreturn_t pcie_ep_irq(int irq, void *data)
{
...
	case PCIE_EP_INT_MHI_A7:
		pci_epc_custom_notify(epc, PCIE_EP_INT_MHI_A7);
		break;
	case PCIE_EP_INT_MHI_Q6:
		pci_epc_custom_notify(epc, PCIE_EP_INT_MHI_Q6);
		break;
	case PCIE_EP_INT_MMIO_WRITE:
		pci_epc_custom_notify(epc, PCIE_EP_INT_MMIO_WRITE);
		break;
...
}
```

EPF
---

```
static int pci_epf_notifier(struct notifier_block *nb, unsigned long val,
			    void *data)
{
...
	case CUSTOM:
		cus_event = data;
		if (cus_event == PCIE_EP_INT_MHI_A7)
			/* Handle MHI A7 event */
		else if (cus_event == PCIE_EP_INT_MHI_Q6)
			/* Handle MHI Q6 event */
		else if (cus_event == PCIE_EP_INT_MMIO_WRITE)
			/* Handle MMIO WRITE event */
		break;
...
}
```

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>