USB: musb: pm: don't rely fully on clock support

Since clock support is optional across processors, don't make the whole
musb pm paths depend upon it.  Just conditionalize the clock accesses.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>