msm: clock-gcc-8974: Add missing sleep_clk to usb_otg device

Define clock lookup entry for usb_otg sleep_clk. This was missed
since usb_otg is normally disabled on 8974 except for a few
board configurations.

Change-Id: I795005fc521ab7d065cf44f916878d9ee81f3a3e
Signed-off-by: Jack Pham <jackp@codeaurora.org>