configs: defconfig for Sipeed Maix in S-mode

Provide a defconfig that can be used to build U-Boot for the Maix boards
running upon OpenSBI.

Update the documentation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
diff --git a/board/sipeed/maix/MAINTAINERS b/board/sipeed/maix/MAINTAINERS
index e7bb9ec..0778af7 100644
--- a/board/sipeed/maix/MAINTAINERS
+++ b/board/sipeed/maix/MAINTAINERS
@@ -4,7 +4,7 @@
 F:	arch/riscv/dts/k210.dtsi
 F:	arch/riscv/dts/k210-maix-bit.dts
 F:	board/sipeed/maix/
-F:	configs/sipeed_maix_bitm_defconfig
+F:	configs/sipeed_maix*_defconfig
 F:	doc/board/sipeed/
 F:	include/configs/sipeed-maix.h
 F:	include/dt-bindings/*/k210-sysctl.h