ARM: dts: stm32: enable stm32mp157's &gpu by default

In the original stm32mp157c.dtsi, the GPU was disabled as some SoC
variants lacked a GPU. We now have separate a dtsi for each SoC
variant and variants without a GPU lack the node altogether.

As we need no board support for using the GPU, enable it by default
and while at it remove the now redundant status = "okay" in existing
board device trees.

Suggested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>