pci: msm: Add API to control pcie link state of NTN3 downstream port

On CPE platform, there is ntn3 pcie switch locates between RC and
Wlan, so the wlan pcie link is separated two parts, RC<->USP and
DSP<->WLAN, when pcie link suspend/resume, the link status of these
two buses is different, when link suspend the RC<->USP is not powered
off due to the existence of Ethernet device under same pcie switch.
For the case of cnss driver to power on/off wlan soc when pcie link
is still turned on, to avoid AER issue, before power on wlan soc cnss
driver needs to disable DSP<->WLAN link and then enable the link and
wait for link training completion Before powering off wlan soc cnss
driver also needs to disable DSP<->WLAN link at first. Function
msm_pcie_ntn3_dsp_link_control() is called by cnss driver to
enable(with link train)/disable DSP<->WLAN link when RC<->USP is on.

Change-Id: I8236b0786a14d6fb6c11805f24407edaa24e9836
Signed-off-by: Harrison Meng <quic_hmeng@quicinc.com>