ARM: dts: at91: sama7g5: add flx8 and required nodes

Add Flexcom8 node with required referenced nodes as phandles.
Since Flexcom8 is present in Linux, take the node exactly as-is from Linux.
Some nodes are referenced in Linux as phandles, the dma and the gic.
Add them as well to the file, even if they are unused by Uboot.
This is a step towards having the U-boot DT equivalent with the DT in Linux.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>