pstore/ram: remove the power of buffer size limitation

There doesn't appear to be any reason for the overall pstore RAM buffer to
be a power of 2 size, so remove it. The individual console, ftrace and oops
buffers are still a power of 2 size.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>