x86: Allow listing MTRRs in SPL

x86: Allow listing MTRRs in SPL

Move MTRR-listing code into a common file so it can be used from SPL.
Update the 'mtrr' command to call it.

Use this in SPL just before adjusting the MTRRs, so we can see the state
set up by the board. Only show it when debug is enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>