soc: arm: add support for FlexSPI1 clock configuration for NXP RT5xx

Add support for configuring FlexSPI1 clock speed to RT5xx soc
initialization, so that memory present on FlexSPI1 can be accessed.
Note that FlexSPI1 is referred to as FlexSPI2 in the dts files for this
SOC.

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