soc: arm: Enable FlexSPI flash driver on i.MX RT family

Enables the FlexSPI flash driver on the i.MX RT SoC family and
configures the peripheral clocks accordingly. We are careful to only
configure the peripheral clocks if we are not executing in place from
the FlexSPI flash.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>