ARM: dts: i.MX51: Update CPU node

This patch updates i.MX51 CPU node:
- Alias for CPU is added to allow using this node in the parent DTS files.
- Removed useless "clock_names" property.
- "clock-latency" value increased to safe using with 32 kHz OSC.
- Defined operating points voltages and "voltage tolerance" properties.
  Values are safe for both commercial and industrial CPU variants.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>