firmware: qcom_scm: Enable legacy convention in qcom_scm-64.c

Enable legacy calling convention in qcom_scm-64. Add enum to represent
current calling convention being used. Update is_call_available to use
legacy function numbers when using legacy convention.

Change-Id: I4aa1364f219d4db68c03023e63009a53326ecca5
Signed-off-by: Elliot Berman <eberman@codeaurora.org>