net-next: mediatek: add platform data to adapt into various hardware

This patch is the preparation patch in order to adapt into various
hardware through adding platform data which holds specific characteristics
among MediaTek SoCs and introducing the unified clock handler for those
distinct clock requirements depending on different features such as
TRGMII and SGMII getting support on the target SoC. And finally, add
enhancement with given the generic description for Kconfig and remove the
unnecessary machine type dependency in Makefile.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>