clk: zynq: Force CPU_2X clock to be ungated

The CPU_2X clock does not have a classical in-kernel user, but is,
amongst other things, required for OCM and debug access. Make sure this
clock is not mistakenly disabled during boot up by enabling it in the
platform's clock driver.

Cc: stable@vger.kernel.org # 3.11+
Fixes: 0ee52b15 'clk: zynq: Add clock controller driver'
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>