firmware: qcom_scm-64: Make SMCCC macros less magical

Improve understandability of SMCCC macros as they are all functions of
how many arguments can be shoved in registers and how many SCM arguments
are supported.

Change-Id: Iac0378fc4c5e20943a64b98223d53caa4955dd53
Signed-off-by: Elliot Berman <eberman@codeaurora.org>