kunit: qemu_configs: powerpc: Explicitly enable CONFIG_CPU_BIG_ENDIAN=y

kunit: qemu_configs: powerpc: Explicitly enable CONFIG_CPU_BIG_ENDIAN=y

The configuration generated by kunit ends up with big endian.
A new kunit configuration for little endian is to be added.
To make the difference clearer spell out the endianness in the kunit
reference config.

Link: https://lore.kernel.org/r/20250415-kunit-ppc-v1-1-f5a170264147@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>