msm: pcie: support L1ss for both RC0 and RC1

Different PCIe endpoints may have different offsets for L1 substates (L1ss)
control to enable L1ss. So add the support here to enable L1ss for both
endpoints of RC0 and RC1.

Change-Id: Iad3ca487358525712d1711fba54c6a4964eb7f8e
Signed-off-by: Yan He <yanhe@codeaurora.org>