The window of the Aspeed AST2400 SMC Controllers to map chips on the
AHB Bus has a 256MB size. The full window range is

    [ 0x20000000 - 0x2FFFFFFF ] for the FMC controller
    [ 0x30000000 - 0x3FFFFFFF ] for the SPI controller

This change requires CONFIG_VMSPLIT_2G to be set.
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>