soc: nxp_kinetis: Refactor flash configuration field to a common place

Refactors the kinetis flash configuration field so it can be shared
across all kinetis SoCs. Defaults are overridden for the k8x and ke1xf
series to preserve values used prior to this refactoring.

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