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>
diff --git a/arch/x86/dts/minnowmax.dts b/arch/x86/dts/minnowmax.dts
index 68e0510..466309f 100644
--- a/arch/x86/dts/minnowmax.dts
+++ b/arch/x86/dts/minnowmax.dts
@@ -206,7 +206,7 @@
 					memory-map = <0xff800000 0x00800000>;
 					rw-mrc-cache {
 						label = "rw-mrc-cache";
-						reg = <0x006f0000 0x00010000>;
+						reg = <0x005f0000 0x00010000>;
 					};
 				};
 			};