drivers: clock: siwx91x: Update QSPI clock source to INTF_PLL

Doing this increases the QSPI peripheral clock from 40 MHz to 80 MHz.
This improves flash access throughput and overall performance
on siwx91x devices, as QSPI is used for external flash operations.

Signed-off-by: Sai Santhosh Malae <Santhosh.Malae@silabs.com>
(cherry picked from commit 2ed508222dbf6572d7fc6a88b3f0c7674f339a7b)

drivers/clock_control/clock_control_silabs_siwx91x.c

diff --git a/drivers/clock_control/clock_control_silabs_siwx91x.c b/drivers/clock_control/clock_control_silabs_siwx91x.c
index 2b37cb3..66121cc 100644
--- a/drivers/clock_control/clock_control_silabs_siwx91x.c
+++ b/drivers/clock_control/clock_control_silabs_siwx91x.c
@@ -251,6 +251,9 @@
 	/* Use interface PLL at configured frequency as peripheral clock */
 	sl_si91x_clock_manager_set_pll_freq(INFT_PLL, INTF_PLL_FREQUENCY, PLL_REF_CLK_VAL_XTAL);
 
+	/* Change the QSPI clock source to INTF_PLL */
+	RSI_CLK_QspiClkConfig(M4CLK, QSPI_INTFPLLCLK, 0, 0, 1);
+
 	/* FIXME: Currently the clock consumer use clocks without power on them.
 	 * This should be fixed in drivers. Meanwhile, get the list of required
 	 * clocks using DT labels.