if_cgem: Cleanup compatible and add new ones

• Remove cdns,gem, it's the generic binding but for all platform that include this one we need specific drivers setup so remove it.
• Remove cdns,macb, it's the generic binding for Atmel AT91 which we don't suport
• Remove cadence,gem, it's not an official binding and seems to be only used in some obscure ARM11 SoC.
• Note that the cdns,zynq* are deprecated
• Add the new Xilinx compatible for zynq and zynqmp

Reviewed-by: mhorne
Tested-by: skibo, Milan Obuch <bsd@dino.sk>
Differential Revision: https://reviews.freebsd.org/D41856
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit bdbbbbb32104569fccd786d9cc07d17f6231a713)