net: smsc91x: add u16 workaround for pxa platforms

Add a workaround for mainstone, idp and stargate2 boards, for u16 writes
which must be aligned on 32 bits addresses.
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Jeremy Linton <jeremy.linton@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/smsc-lan91c111.txt

@@ -13,3 +13,5 @@ Optional properties:
   16-bit access only.
 - power-gpios: GPIO to control the PWRDWN pin
 - reset-gpios: GPIO to control the RESET pin
+- pxa-u16-align4 : Boolean, put in place the workaround the force all
+		   u16 writes to be 32 bits aligned