msm: pcie: improve link recovery mechanism

Improve the mechanism for PCIe link down handling, and update the
the interface between PCIe device drivers and PCIe bus driver for
link recovery.

CRs-fixed: 678347
Change-Id: I48e2d77f26f3f9f29c81cf98a2b7ab5a477138f4
Signed-off-by: Yan He <yanhe@codeaurora.org>