arm64: dts: imx8mq-librem5-r3: Mark buck3 as always on

Commit 99e71c02 ("arm64: dts: imx8mq-librem5: Don't mark buck3 as always on")
removed always-on marking from GPU regulator, which is great for power
saving - however it introduces additional i2c0 traffic which can be deadly
for devices from the Dogwood batch.

To workaround the i2c0 shutdown issue on Dogwood, this commit marks
buck3 as always-on again - but only for Dogwood (r3).

Signed-off-by: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>