x86: mtrr: Enhance 'mtrr' command to list MTRRs on any CPU

Update this command so it can list the MTRRs on a selected CPU. If
'-c all' is used, then all CPUs are listed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>