clk: mediatek: Properly include clk.h

We don't need to include clk.h in header files, just forward
declare struct clk here. This leads us to a few places where the
include of clk.h was missing in C files. Add them.

Cc: James Liao <jamesjj.liao@mediatek.com>
Cc: Henry Chen <henryc.chen@mediatek.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>