ARM: dts: dove: add DT GPU support

Add DT support for the Vivante GC600 GPU on Marvell Dove platforms.
These nodes default to being disabled unless a platform decides they
should be enabled.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>