soc: arm: nxp_imx: fix flexspi frequency setting for iMXRT11xx SOC

Commit a10fee2d (drivers: clock_control: ccm_rev2: add support for
reclocking FlexSPI) introduced the ability to set the FlexSPI
clock frequency at runtime on RT11xx series SOCs. However, this
implementation resulted in the clock frequency being rounded up, not
down. This can result in flash clock frequency violations on some
flash parts, causing the system to crash when running in XIP mode.

Fixes #69088

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>