ARM: imx: define struct clk_gate2 on our own

The imx clk-gate2 driver implements an i.MX specific gate clock, which
has two bits controlling the gate states.  While this is a completely
separate gate driver from the common clk-gate one, it reuses the common
clk_gate structure.  Such reusing makes the extending of clk_gate2
clumsy.  Let's define struct clk_gate2 on our own to make the driver
independent of the common clk-gate one, and ease the clk_gate2 extending
at a later time.

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