usb: phy: msm: Make phy_reset clk and reset line optional.

This patch makes the phy reset clk and reset line optional as this clk
is not available on boards like IFC6410 with APQ8064.

phy-reset clk is only used as argument to the mach level callbacks, so
this patch adds condition before clk_get calls so that the driver
wouldn't fail on SOCs which do not have this support.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>