msm: pcie: add the option for endpoint latency

PCIe endpoint may require some latency after power on, before PCIe
Root Complex (RC) de-asserts PERST. Add the option here to support
this latency requirement.

Change-Id: I45df5a2306b6c3681fcd24af571552d346c6d87e
Signed-off-by: Yan He <yanhe@codeaurora.org>