sgi575: Fix GCC 7 -Wformat-truncation error

GCC 7 introduces the -Wformat-truncation warning and associated
heuristics. This warning identifies potentially unexpected string
truncation issues by statically range-checking the vararg inputs to
snprintf().

For us, GCC seems to be unable to reliably identify the bounds of the
inputs (cluster_idx, core_idx) even though we have static guarantees in
place as those guarantees are made in a separate compilation unit from
the one in which the error occurs.

This patch allows the compiler to inline the cluster/core count
functions to ensure it can properly ascertain the proper bounds on the
inputs to snprintf().

Change-Id: I23447732b579f0c02d739744c59cbeb5bf774d6f
Signed-off-by: Chris Kay <chris.kay@arm.com>