usb: dwc3-msm: Support phy sleep clock reset

Some targets require explicit reset of the phy sleep
clock during initialization. Add a device tree flag
to indicate when is needed.
Also prefer the clock name "phy_sleep_clk" when
calling clk_get(), while supporting "sleep_a_clk"
for older targets.

Change-Id: I18c4d9c6bd426a1a203538fe57592119f2bd95f7
Signed-off-by: Maya Erez <merez@codeaurora.org>