add flash/nor/core.[ch]

add flash/nor/core.[ch]

The newly moved flash TCL routines access the internals of the module
too much.  Fix the layering issues by adding new core NOR flash APIs:

<flash/nor/core.h>:
  - flash_driver_find_by_name() - self-descriptive

<flash/nor/imp.h>:
  - flash_bank_add()            - encapsulates adding banks to bank list
  - flash_bank_list()           - encapsulates retreiving bank list

This allows the externs in flash/nor/imp.h to be removed, and
these mechanisms may now be re-used by other flash module code.