ARM64 / ACPI: Ignore the return error value of acpi_map_gic_cpu_interface()

MADT scanning will stop when it gets an error from the handler,
acpi_map_gic_cpu_interface(), on arm64.  However, we need to
find all of the enabled CPUs so that SMP initialization can work
properly.  So, if an error occurs in this case, ignore it for
now so that we can find all of the enabled CPUs.

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