ARM: dts: aspeed: palmetto: Fix flash_memory region

The flash_memory region was incorrect and exceeds AST2400's RAM range.
Fix it by putting it before coldfire region, and aligned with 32MiB.

Signed-off-by: Lei YU <mine260309@gmail.com>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>