net: cpmac: remove driver to prepare for platform removal

AR7 is going to be removed from the Kernel, so remove its networking
support in form of the cpmac driver. This allows us to remove the
platform because this driver includes a platform specific header.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/all/20230922061530.3121-6-wsa+renesas@sang-engineering.com/


Signed-off-by: Jakub Kicinski <kuba@kernel.org>