drivers: clock_control: add support for FlexSPI reclock on NXP iMX RT10XX

Add support for reclocking the FlexSPI on NXP iMX RT10XX. This
functionality requires an SOC specific clock function to set
the clock rate, since the FlexSPI must be reset directly
before applying the new clock frequency.

Note that all clock constants are defined in this commit, since the
memc flexspi driver now depends on a clock node being present.

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