mtd: aspeed: add memory controllers for the Aspeed AST2400 SoC

This driver adds mtd support for the Aspeed AST2400 SoC static memory
controllers:

 * New Static Memory Controller (referred as FMC)
   . BMC firmware
   . AST2500 compatible register set
   . 5 chip select pins (CE0 ∼ CE4)
   . supports NOR flash, NAND flash and SPI flash memory.

 * SPI Flash Controller (SPI)
   . host Firmware
   . slightly different register set, between AST2500 and the legacy
     controller
   . supports SPI flash memory
   . 1 chip select pin (CE0)

The legacy static memory controller (referred as SMC) is not
supported, as well as types other than SPI.

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