ARM: dts: msm: Make lnbb clk as ref_clk_src for mdm9640

commit accb3bb5 ("usb: phy: Add ref_clk_src to PHY driver")
defines lnbb clk as ref_clk_src. Other targets define
different clk as ref_clk. Hence define lnbb clk as
ref_clk_src and make ref_clk as optional clk for mdm9640.

Change-Id: I1d92bbbcafe14b72549b9e44d6fd2583a25b218f
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>