csky: fix syscache.c fallthrough warning

This case of the switch statement falls through to the following case.
This appears to be on purpose, so declare it as OK.

../arch/csky/mm/syscache.c: In function '__do_sys_cacheflush':
../arch/csky/mm/syscache.c:17:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   17 |   flush_icache_mm_range(current->mm,
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   18 |     (unsigned long)addr,
      |     ~~~~~~~~~~~~~~~~~~~~
   19 |     (unsigned long)addr + bytes);
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../arch/csky/mm/syscache.c:20:2: note: here
   20 |  case DCACHE:
      |  ^~~~

Fixes: 997153b9 ("csky: Add flush_icache_mm to defer flush icache all")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: linux-csky@vger.kernel.org
Cc: Arnd Bergmann <arnd@arndb.de>