ARM: zynq: dt: Add a fixed regulator for CPU voltage

To silence the warning
	cpufreq_cpu0: failed to get cpu0 regulator: -19
from the cpufreq driver regarding a missing regulator,
add a fixed regulator to the DT.
Zynq does not support voltage scaling and the CPU rail should always be
supplied with 1 V, hence it is added in the SOC-level dtsi.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>