x86: mtrr: Update 'mtrr' to allow setting MTRRs on any CPU

Add a -c option to mtrr to allow any CPU to be updated with this command.

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>