samples: power: Remove mention of specific versions in README

The README was referring to a specific version of the boot loader.
Since the boot loader version is expected to change in future
and Zephyr code would also correspond to that version, it is better
to not mention any specific version. Instead, the requirement
is to use the latest version of the boot loader.

Change-Id: I04082eec4af16426f6456702c521e8f61bd4e0df
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>