DT: hwspinlock: Add binding documentation for Qualcomm hwmutex

Add binding documentation for the Qualcomm Hardware Mutex.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Kumar Gala <galak@codeaurora.org>
Reviewed-by: Andy Gross <agross@codeaurora.org>
Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>