nds32: Fix building error when CONFIG_FREEZE is enabled.

To include kernel/Kconfig.freezer to make sure the dependency between
CONFIG_CGROUP_FREEZER and CONFIG_FREEZER

It will cause building error when I make allmodconfig.

kernel/cgroup/freezer.c: In function 'freezer_css_online':
kernel/cgroup/freezer.c:116:15: error: 'system_freezing_cnt' undeclared (first use in this function)
   atomic_inc(&system_freezing_cnt);
               ^~~~~~~~~~~~~~~~~~~
kernel/cgroup/freezer.c:116:15: note: each undeclared identifier is reported only once for each function it appears in
kernel/cgroup/freezer.c: In function 'freezer_css_offline':
kernel/cgroup/freezer.c:137:15: error: 'system_freezing_cnt' undeclared (first use in this function)
   atomic_dec(&system_freezing_cnt);
               ^~~~~~~~~~~~~~~~~~~
kernel/cgroup/freezer.c: In function 'freezer_attach':
kernel/cgroup/freezer.c:181:4: error: implicit declaration of function 'freeze_task' [-Werror=implicit-function-declaration]
    freeze_task(task);
    ^~~~~~~~~~~
kernel/cgroup/freezer.c: In function 'freezer_apply_state':
kernel/cgroup/freezer.c:360:16: error: 'system_freezing_cnt' undeclared (first use in this function)
    atomic_inc(&system_freezing_cnt);
                ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>