qcom: scm: Support register x6 to pass the session id

Non-atomic scm call which could be interrupted, trustzone will store
the session id in a register(x6) which will be used when trustzone resumes
the call.
To avoid x6 being used by compiler, HLOS now uses it to send a zero before
making scm call.
This is the same change as in the 32bit scm call.

Change-Id: If7a3ee28bdbf22acf447531603819a6f4f1603ca
Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>