x86: som-db5800-som-6867: 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.

Fixes: e23cae30 ("x86: som-db5800-som-6867: Adjust CONFIG_TEXT_BASE")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>