firmware: qcom_scm: Add SCM call to perform warm reboot

An SCM call can be made instead of using PSCI SYS_RESET2. Use this SCM
call instead as PSCI driver does not have support to do vendor reset
types.

Change-Id: I07ccdcb408df0ef8a6a6c09f744ccff6345f0dea
Signed-off-by: Elliot Berman <eberman@codeaurora.org>