ARM64 / ACPI: make acpi_map_gic_cpu_interface() as void function

Since the only caller of acpi_parse_gic_cpu_interface() doesn't
need the return value, make it have a void return type to avoid
introducing subtle bugs, and update the comments of the function
accordingly.

Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>