mtd: spinand: Refactor spinand_init* functions

mtd: spinand: Refactor spinand_init* functions

No functional changes, just some refactoring to better match linux
kernel driver.

changes:
 * move spinand configuration reading out from spinand_init_cfg_cache()
   to separate function spinand_read_cfg()
 * move spinand flash initialization to separate function
   spinand_init_flash()
 * move direct mapping initialization to the end of spinand_init()

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>