boards: qemu_cortex_a53: move MAX_THREAD_BYTES to Kconfig

This moves CONFIG_MAX_THREAD_BYTES from the board's defconfig
file into the Kconfig file. This is to get rid of the cmake
warning about CONFIG_MAX_THREAD_BYTES being assigned value
but got the value ''. This is due to CONFIG_USERSPACE being
disabled, where some tests explicitly do so. This is simply
done to avoid confusion when running those tests manually.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>