soc: nxp: Centralize flexspi related configuration

Currently this code related to how to configure the
flash size and address when using flexspi to XIP is copy
pasted in all sort of places and ways all over the tree,
let's clean this up and have single point of control over
this configuration.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>