[ARM] DNS323: Initialise 88F5182 correctly

The 88F5182 found in the DNS-323 rev B1 (and some other devices, such
as the CH3SNAS) require different initialisation of the SATA
controller and MPP registers.

Tested on a DNS-323 rev B1.

Signed-off-by: Matt Palmer <mpalmer@hezmatt.org>