spi: spi-nxp-fspi: set back to dll override mode when clock rate < 100MHz

spi: spi-nxp-fspi: set back to dll override mode when clock rate < 100MHz

Preparation of supportting DTR mode. In nor suspend, driver will disable
DTR mode, and enable DTR mode back in nor resume. This require the flexspi
driver has the ability to set back to dll override mode in STR mode when
clock rate < 100MHz.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20250917-flexspi-ddr-v2-2-bb9fe2a01889@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>