msm: pcie: add PCIe sleep clock to list of clocks

PCIe PHY and core require the need of a sleep clock for
certain features. Add this clock to the list of PCIe
clocks so PCIe bus driver can enable/disable it.

Change-Id: I8930d65d9070b98f6c7e49b98ad4db09d539ce28
Signed-off-by: Tony Truong <truong@codeaurora.org>