clk: imx: clk-pllv3: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/clk/imx/clk-pllv3.c: In function ‘imx_clk_pllv3’:
drivers/clk/imx/clk-pllv3.c:446:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
   pll->div_shift = 1;
   ~~~~~~~~~~~~~~~^~~
drivers/clk/imx/clk-pllv3.c:447:2: note: here
  case IMX_PLLV3_USB:
  ^~~~
drivers/clk/imx/clk-pllv3.c:453:21: warning: this statement may fall through [-Wimplicit-fallthrough=]
   pll->denom_offset = PLL_IMX7_DENOM_OFFSET;
                     ^
drivers/clk/imx/clk-pllv3.c:454:2: note: here
  case IMX_PLLV3_AV:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Fixes: b4a4cb5a ("clk: imx: correct i.MX7D AV PLL num/denom offset")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>