DT: bindings: add missing Marvell Kirkwood SoC documentation

Marvell Kirkwood SoC binding have not yet been documented. Add the
documentation including the supported SoCs and boards.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/marvell,kirkwood.txt

+Marvell Kirkwood SoC Family Device Tree Bindings
+------------------------------------------------
+
+Boards with a SoC of the Marvell Kirkwook family, eg 88f6281
+
+* Required root node properties:
+compatible: must contain "marvell,kirkwood"
+
+In addition, the above compatible shall be extended with the specific
+SoC. Currently known SoC compatibles are:
+
+"marvell,kirkwood-88f6192"
+"marvell,kirkwood-88f6281"
+"marvell,kirkwood-88f6282"
+"marvell,kirkwood-88f6283"
+"marvell,kirkwood-88f6702"
+"marvell,kirkwood-98DX4122"
+
+And in addition, the compatible shall be extended with the specific
+board. Currently known boards are:
+
+"buffalo,lschlv2"
+"buffalo,lsxhl"
+"buffalo,lsxl"
+"dlink,dns-320"
+"dlink,dns-320-a1"
+"dlink,dns-325"
+"dlink,dns-325-a1"
+"dlink,dns-kirkwood"
+"excito,b3"
+"globalscale,dreamplug-003-ds2001"
+"globalscale,guruplug"
+"globalscale,guruplug-server-plus"
+"globalscale,sheevaplug"
+"globalscale,sheevaplug"
+"globalscale,sheevaplug-esata"
+"globalscale,sheevaplug-esata-rev13"
+"iom,iconnect"
+"iom,iconnect-1.1"
+"iom,ix2-200"
+"keymile,km_kirkwood"
+"lacie,cloudbox"
+"lacie,inetspace_v2"
+"lacie,laplug"
+"lacie,netspace_lite_v2"
+"lacie,netspace_max_v2"
+"lacie,netspace_mini_v2"
+"lacie,netspace_v2"
+"marvell,db-88f6281-bp"
+"marvell,db-88f6282-bp"
+"marvell,mv88f6281gtw-ge"
+"marvell,rd88f6281"
+"marvell,rd88f6281"
+"marvell,rd88f6281-a0"
+"marvell,rd88f6281-a1"
+"mpl,cec4"
+"mpl,cec4-10"
+"netgear,readynas"
+"netgear,readynas"
+"netgear,readynas-duo-v2"
+"netgear,readynas-nv+-v2"
+"plathome,openblocks-a6"
+"plathome,openblocks-a7"
+"raidsonic,ib-nas6210"
+"raidsonic,ib-nas6210-b"
+"raidsonic,ib-nas6220"
+"raidsonic,ib-nas6220-b"
+"raidsonic,ib-nas62x0"
+"seagate,dockstar"
+"seagate,goflexnet"
+"synology,ds109"
+"synology,ds110jv10"
+"synology,ds110jv20"
+"synology,ds110jv30"
+"synology,ds111"
+"synology,ds209"
+"synology,ds210jv10"
+"synology,ds210jv20"
+"synology,ds212"
+"synology,ds212jv10"
+"synology,ds212jv20"
+"synology,ds212pv10"
+"synology,ds409"
+"synology,ds409slim"
+"synology,ds410j"
+"synology,ds411"
+"synology,ds411j"
+"synology,ds411slim"
+"synology,ds413jv10"
+"synology,rs212"
+"synology,rs409"
+"synology,rs411"
+"synology,rs812"
+"usi,topkick"
+"usi,topkick-1281P2"
+"zyxel,nsa310"
+"zyxel,nsa310a"