arm64: dts: imx8mp: Add GPT blocks

The i.MX8MP includes the same GPT blocks as the i.MX6DL. Add all 6
instances.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>