ARM: dts: aspeed: ast2600evb: Use custom flash layout

The AST2600 u-boot and kernel images have outgrown the OpenBMC layout.
While BMC machines use 128MB SPI NOR chips, we only have 64MB on the EVB
so use a layout that has a smaller region for the ro and rw filesystems.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>