clk: Add support for shared gates ...

Sometimes a single software control knob controls multiple gates
in hardware. This patch adds support for shared gates which help
coping this situation. The first gate is registered with the hardware
gate as usual, the others are registered as shared gates which does
not have hardware control itself, but only switches the real hardware
gate.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>