serial: qcom-geni: Add DFS clock mode support to GENI UART driver

serial: qcom-geni: Add DFS clock mode support to GENI UART driver

GENI UART driver currently supports only non-DFS (Dynamic Frequency
Scaling) mode for source frequency selection. However, to operate correctly
in DFS mode, the GENI SCLK register must be programmed with the appropriate
DFS index. Failing to do so can result in incorrect frequency selection

Add support for Dynamic Frequency Scaling (DFS) mode in the GENI UART
driver by configuring the GENI_CLK_SEL register with the appropriate DFS
index. This ensures correct frequency selection when operating in DFS mode.

Replace the UART driver-specific logic for clock selection with the GENI
common driver function to obtain the desired frequency and corresponding
clock index. This improves maintainability and consistency across
GENI-based drivers.

Signed-off-by: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250903063136.3015237-1-viken.dadhaniya@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>