arm: dts: sync am33xx gpio nodes with Linux 5.9-rc7

Move gpio controller nodes from am33xx.dtsi to am33xx-l4.dtsi. Now both
files are more similar to those found in Linux 5.9-rc7. To be fully
operational as before it was also necessary to fix the am33xx-clock.dtsi
file. It too is now almost the same as the Linux kernel.

Signed-off-by: Dario Binacchi <dariobin@libero.it>