dts: gigadevice: remove deprecated device_type property

According to DT spec, device_type property is deprecated (ref. 0.3 spec,
2.3.11):

> The device_type property was used in IEEE 1275 to describe the device’s
FCode programming model. Because DTSpec does not have FCode, new use
of the property is deprecated, and it should be included only on cpu and
memory nodes for compatibility with IEEE 1275–derived devicetrees.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>