kunit: qemu_configs: sparc: Explicitly enable CONFIG_SPARC32=y

The configuration generated by kunit ends up with a 32bit configuration.
A new kunit configuration for 64bit is to be added.
To make the difference clearer spell out the variant in the kunit
reference config.

Link: https://lore.kernel.org/r/20250415-kunit-qemu-sparc64-v1-1-253906f61102@linutronix.de


Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>