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 <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/db8500-prcmu.c

···
 	switch (divsel) {
 	case PRCM_DSI_PLLOUT_SEL_PHI_4:
 		div *= 2;
+		/* Fall through */
 	case PRCM_DSI_PLLOUT_SEL_PHI_2:
 		div *= 2;
+		/* Fall through */
 	case PRCM_DSI_PLLOUT_SEL_PHI:
 		return pll_rate(PRCM_PLLDSI_FREQ, clock_rate(PRCMU_HDMICLK),
 			PLL_RAW) / div;