exynos4210_gic: Suppress gcc9 format-truncation warnings

exynos4210_gic: Suppress gcc9 format-truncation warnings

exynos4210_gic_realize() prints the number of cpus into some temporary
buffers, but it only allows 3 bytes space for it.  That's plenty:
existing machines will only ever set this value to EXYNOS4210_NCPUS
(2).  But the compiler can't always figure that out, so some[*] gcc9
versions emit -Wformat-truncation warnings.

We can fix that by hinting the constraint to the compiler with a
suitably placed assert().

[*] The bizarre thing here, is that I've long gotten these warnings
    compiling in a 32-bit x86 container as host - Fedora 30 with
    gcc-9.2.1-1.fc30.i686 - but it compiles just fine on my normal
    x86_64 host - Fedora 30 with and gcc-9.2.1-1.fc30.x86_64.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
[PMM: deleted stray blank line]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>