USB: HSIC SMSC HUB: Fix device tree style problems

Add the missing documentation for some device tree properties. Fix
the regulator supply naming convention.  The hub-vbus supply is
optional. Don't call regualtor_get on this supply unconditionally.
This leads to unnecessary multiple probe deferrals.

Change-Id: I866bfb9d87ad274030b7997461e89c6bc42f6d98
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>