urukul: update clock tree symbols

Represent multiplications with rectangles, and make multiplication operation implicit.

src/hw/urukul.md

@@ -46,15 +46,13 @@ XO -->|clk_sel:0| CLKMUX
 SMA -->|clk_sel:1| CLKMUX
 MMCX -->|clk_sel:2| CLKMUX
 
-CLKDIV(("÷
-    divider"))
+CLKDIV[1/divider]
 CLKMUX -->|refclk| CLKDIV
 
 REFCLK@{ shape: f-circ }
 CLKDIV --> REFCLK
 
-PLL(("x
-    pll_n"))
+PLL[pll_n]
 REFCLK --> PLL
 
 PLL_EN@{ shape: f-circ }