clk: mediatek: Add initial common clock support for Mediatek SoCs.

This patch adds common clock support for Mediatek SoCs, including plls,
muxes and clock gates.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Signed-off-by: Henry Chen <henryc.chen@mediatek.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[sboyd@codeaurora.org: Squelch checkpatch warning in clk-mtk.h]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>