MIPS: Fix fall-through warnings for Clang

Fix the following fallthrough warnings:

arch/mips/mm/tlbex.c:1386:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
arch/mips/mm/tlbex.c:2173:3: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

arch/mips/mm/tlbex.c

@@ -1383,6 +1383,7 @@ static void build_r4000_tlb_refill_handler(void)
 	switch (boot_cpu_type()) {
 	default:
 		if (sizeof(long) == 4) {
+		fallthrough;
 	case CPU_LOONGSON2EF:
 		/* Loongson2 ebase is different than r4k, we have more space */
 			if ((p - tlb_handler) > 64)
@@ -2169,6 +2170,7 @@ static void build_r4000_tlb_load_handler(void)
 		default:
 			if (cpu_has_mips_r2_exec_hazard) {
 				uasm_i_ehb(&p);
+			fallthrough;
 
 		case CPU_CAVIUM_OCTEON:
 		case CPU_CAVIUM_OCTEON_PLUS: