oxnas: add support for 2nd S-ATA port to sata_oxnas driver

oxnas: add support for 2nd S-ATA port to sata_oxnas driver

similar to mv_sata, use nr-ports attribute from device tree.
import and adapt locking code from vendor GPL sources.
add dma controller handling, it may be used in future to avoid
full core resets similar to the vendor SDK's "progressive cleanup"
function.

this is still very dirty and aimed to first of all do things
quite exactly like the reference code. and it somehow works.
obviously there is lots of room for improvement :)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43598 3c298f89-4303-0410-b956-a3cf2f4a3e73