[POWERPC] 4xx: Convert Seqouia flash mappings to new binding

A new binding for flash devices was recently introduced.  This updates the
Sequoia DTS to use the new binding and enabled MTD in the defconfig.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Stefan Roese <sr@denx.de>