allow flash/nand banks commands to accept names

allow flash/nand banks commands to accept names

Add get_flash_bank_by_name (and get_nand_device_by_name) helpers
to retrieves struct flash_bank * (struct nand_device *) given a
driver name and an (optional) driver-specific bank index.

These are used to extend flash_command_get_bank_by_num (and
nand_command_get_device_by_num) to allow all flash (nand) commands to
reference defined banks by name, not just by number.

To avoid some code duplication, add the flash/common.[ch] files to hold
functionality common to both types driver.  The first two methods are
helpers for the above routines to find a bank specified by a "name" or
"name.index" string.  get_flash_name_index() finds the '.index' portion,
while flash_driver_name_matches() performs the string portion matching.