wiseconnect: Drop use of pow()

pow() is not defined when Zephyr is built with minimal libc. There is
only one occurence of the pow() function in Wiseconnect.

Since Wiseconnect only use pow() to compute power of 2, it can be
replaced by a bit shift.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>

wiseconnect/components/device/silabs/si91x/mcu/drivers/systemlevel/src/rsi_pll.c

@@ -624,7 +624,7 @@ rsi_error_t clk_set_i2s_pll_freq(const M4CLK_Type *pCLK, uint32_t i2sPllFreq, ui
   FCW   = (float)Fdco / (float)fref;
   M     = (uint16_t)FCW;
   frac  = (FCW - M);
-  FCW_F = (uint16_t)(frac * pow(2, 14));
+  FCW_F = (uint16_t)(frac * (1 << 14)); // (1 << 14) == pow(2, 14)
   if (Fdco == I2S_DCO_FREQ1) {
     FCW_F = (FCW_F + 1);
   }