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/configs/sipeed_maix_smode_defconfig b/configs/sipeed_maix_smode_defconfig
new file mode 100644
index 0000000..2516bb7
--- /dev/null
+++ b/configs/sipeed_maix_smode_defconfig
@@ -0,0 +1,10 @@
+CONFIG_RISCV=y
+CONFIG_SYS_TEXT_BASE=0x80020000
+CONFIG_TARGET_SIPEED_MAIX=y
+CONFIG_ARCH_RV64I=y
+CONFIG_RISCV_SMODE=y
+CONFIG_STACK_SIZE=0x100000
+# CONFIG_NET is not set
+# CONFIG_INPUT is not set
+# CONFIG_DM_ETH is not set
+# CONFIG_EFI_UNICODE_CAPITALIZATION is not set