firmware: qcom_scm: Merge legacy and SMCCC conventions

Copy/paste legacy SCM driver into qcom_scm-64 with the following notes:
- Renamed qcom_scm_call in qcom_scm-32 to qcom_scm_call_legacy in the
  copy.
- Renamed qcom_scm_call_atomic in qcom_scm 32 to
  qcom_scm_call_atomic_legacy in the copy.
- Rename __qcom_scm_call_smccc to qcom_scm_call_smccc.
- Filled in implementations set_cold_boot_addr, set_warm_boot_addr, and
  cpu_power_down from qcom_scm-32.c.
- set_dload_mode, io_writel, io_readl now use atomic variants as in
  qcom_scm-32.

Change-Id: Ifa72ded7d6150bef08c6a0451d422b0782e8320a
Signed-off-by: Elliot Berman <eberman@codeaurora.org>