mfd: db8500-prcmu: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/mfd/db8500-prcmu.c: In function 'dsiclk_rate':
drivers/mfd/db8500-prcmu.c:1592:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   div *= 2;
   ~~~~^~~~
drivers/mfd/db8500-prcmu.c:1593:2: note: here
  case PRCM_DSI_PLLOUT_SEL_PHI_2:
  ^~~~
drivers/mfd/db8500-prcmu.c:1594:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   div *= 2;
   ~~~~^~~~
drivers/mfd/db8500-prcmu.c:1595:2: note: here
  case PRCM_DSI_PLLOUT_SEL_PHI:
  ^~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>