PCI: qcom: Add support for MSM8996 PCIe controller

Add support for the MSM8996/APQ8096 PCIe controller.  MSM8996 supports Gen
1/2, one lane, 3 PCIe root complexes with support for MSI and legacy
interrupts, and it conforms to PCI Express Base 2.1 specification.

Add a post_init callback to qcom_pcie_ops, as the PCIe pipe clocks are only
setup after the phy is powered on.  It also adds an ltssm_enable callback
as it is very much different from other supported SoCs in the driver.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Stanimir Varbanov <svarbanov@mm-sol.com>