clk/sophgo: Using BUG() instead of unreachable() in mmux_get_parent_id()

In general it's a good idea to avoid using bare unreachable() because it
introduces undefined behavior in compiled code. but it caused a compilation warning,
Using BUG() instead of unreachable() to resolve compilation warnings.

Fixes the following warnings:
    drivers/clk/sophgo/clk-cv18xx-ip.o: warning: objtool: mmux_round_rate() falls through to next function bypass_div_round_rate()

Fixes: 80fd61ec46 ("clk: sophgo: Add clock support for CV1800 SoC")
Signed-off-by: Li Qiang <liqiang01@kylinos.cn>
Link: https://lore.kernel.org/r/c8e66d51f880127549e2a3e623be6787f62b310d.1720506143.git.liqiang01@kylinos.cn
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/sophgo/clk-cv18xx-ip.c

@@ -613,7 +613,7 @@ static u8 mmux_get_parent_id(struct cv1800_clk_mmux *mmux)
 			return i;
 	}
 
-	unreachable();
+	BUG();
 }
 
 static int mmux_enable(struct clk_hw *hw)