msm: ep-pcie: Use threaded irq for PERST de-assertion handling

When the host de-asserts the PERST, ep-pcie driver needs to bring-up
the link as quickly as possible (within 150ms). Otherwise, the host
may fail to detect the link.

In our current design, we are doing the link initialization and
link training in a kworker thread context. In some scenarios
this context is incurring context switches and scheduling  delays
which is resulting in issues like the host fails to detect the link.

To fix this, perform the link initialization and training in the
highest possible priority (threaedIRQ). With this change, PCIe driver
notifies clients in threadedIRQ context and the clients (Eg MHI)
supposed to invoke the link training in the same context.

Change-Id: I04de4a1dfb3a145b947a2824c6961da70215944b
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>