firmware: qcom_scm: Rename -64 -> -smc, remove -32

- Rename qcom_scm-64.c to qcom_scm-smc as it supports SMC-based calling
  conventions for 32 and 64-bit targets
- Remove qcom_scm-32.c as it has been superceded by qcom_scm-smc.c
- Update Kconfig to select QCOM_SCM_TRANSPORT_SMC on ARM and ARM64
  targets

Change-Id: Iae94a73e0c7cefb9a253230212d01f6126cd0c15
Signed-off-by: Elliot Berman <eberman@codeaurora.org>