arm64: dts: freescale: rename gw7905 to gw75xx

The GW7905 was renamed to GW7500 before production release.

While we typically do not change compatibles, the GW7905 was never
released before its product name was changed to a GW7500.

The use the the 'xx' wildcard is to denote the fact that this
device-tree can support range of board models from GW7500 to GW7599 as
has been done historically with the Gateworks baseboards to support
various build customizatoins based on the same PCB.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>