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)