ARM i.MX: Add common clock support for 2bit gate

This gate consists of two bits:

0b00: clk disabled
0b01: clk enabled in run mode and disabled in sleep mode
0b11: clk enabled

Currently only disabled and enabled are supported. As it's unlikely
that we find something like this in another SoC create a i.MX specific
clk helper for this.

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