msm: pcie: Add the option to support L1ss

L1 sub-states (L1ss) feature of PCIe is optional on targets. Add the
option to enable this feature or not.

Change-Id: I41347e046c81d5e9e9d6baac9324d18b7b83de2b
Signed-off-by: Yan He <yanhe@codeaurora.org>