qcom: scm: Support register r6 to pass the session id

Non-atomic scm call which could be interrupted, trustzone will store
the session id in a register(r6) which will be used when trustzone resumes
the call.
To avoid r6 being used by compiler, HLOS now uses it to send a zero before
making scm call.

Change-Id: I2927efe04fd019d551aae5e6548da3fb5c18b655
Signed-off-by: Taniya Das <tdas@codeaurora.org>