ARM: imx: add shared gate clock support

It's quite common on i.MX that one gate bit controls the gating of
multiple clocks, i.e. this is a shared gate.  The patch adds the
function imx_clk_gate2_shared() for such case.  The clocks controlled
by the same gate bits should call this function with a pointer to a
single share count variable, so that the gate bits will only be
operated on the first enabling and the last disabling of these shared
gate clocks.

Thanks to Gerhard Sittig <gsi@denx.de> for this idea.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>