cosmetic: doc: update README.generic-board

Now MIPS supports 'generic board' feature.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.generic-board b/doc/README.generic-board
index 17da0b9..37c1b03 100644
--- a/doc/README.generic-board
+++ b/doc/README.generic-board
@@ -40,10 +40,11 @@
 ------------------------
 
 If you are unlucky then your architecture may not support generic board.
-The following architectures are supported at the time of writing:
+The following architectures are supported now:
 
    arc
    arm
+   mips
    powerpc
    sandbox
    x86