ARM: 8889/1: dt-bindings: document marvell,ecc-enable binding

Add documentation for the marvell,ecc-enable properties which can be
used to enable ECC on the Marvell aurora cache.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>