mtd: spi-nor: add memory controllers for the Aspeed AST2500 SoC

This driver adds mtd support for the Aspeed AST2500 SoC static memory
controllers :

 * Firmware SPI Memory Controller (FMC)
   . BMC firmware
   . 3 chip select pins (CE0 ~ CE2)
   . supports SPI type flash memory (CE0-CE1)
   . CE2 can be of NOR type flash but this is not supported by the
     driver

 * SPI Flash Controller (SPI1 and SPI2)
   . host firmware
   . 2 chip select pins (CE0 ~ CE1)
   . supports SPI type flash memory

Each controller has a memory range on which it maps its flash module
slaves. Each slave is assigned a memory window for its mapping that
can be changed at bootime with the Segment Address Register.

Each SPI flash slave can then be accessed in two modes: Command and
User. When in User mode, accesses to the memory segment of the slaves
are translated in SPI transfers. When in Command mode, the HW
generates the SPI commands automatically and the memory segment is
accessed as if doing a MMIO.

Currently, only the User mode is supported. Command mode needs a
little more work to check that the memory window on the AHB bus fits
the module size.

Based on previous work from Milton D. Miller II <miltonm@us.ibm.com>

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>