x86: minnowmax: Fix up adjustment of CONFIG_TEXT_BASE

With recent CONFIG_TEXT_BASE changes, there are inconsistencies between
several settings.

Adjust CONFIG_SYS_MONITOR_LEN to allow more code space. Move the MRC
cache out of the way too.

Add documentation on how to make this change safely.

Fixes: 66e2c665f3b6 ("x86: minnowmax: Adjust CONFIG_TEXT_BASE")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
3 files changed