arm: zynq: Add support for cpufreq

arm: zynq: Add support for cpufreq

The generic cpufreq-cpu0 driver can scale the CPU frequency on Zynq
SOCs. Add the required platform device to the BSP and appropriate
OPPs to the dts.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: devicetree@vger.kernel.org
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Michal Simek <michal.simek@xilinx.com>