cpufreq: s3c244x: add fallthrough comments for switch

Apparently nobody has so far caught this warning, I hit it in randconfig
build testing:

drivers/cpufreq/s3c2440-cpufreq.c: In function 's3c2440_cpufreq_setdivs':
drivers/cpufreq/s3c2440-cpufreq.c:175:10: error: this statement may fall through [-Werror=implicit-fallthrough=]
   camdiv |= S3C2440_CAMDIVN_HCLK3_HALF;
          ^
drivers/cpufreq/s3c2440-cpufreq.c:176:2: note: here
  case 3:
  ^~~~
drivers/cpufreq/s3c2440-cpufreq.c:181:10: error: this statement may fall through [-Werror=implicit-fallthrough=]
   camdiv |= S3C2440_CAMDIVN_HCLK4_HALF;
          ^
drivers/cpufreq/s3c2440-cpufreq.c:182:2: note: here
  case 4:
  ^~~~

Both look like the fallthrough is intentional, so add the new
"fallthrough;" keyword.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>