riscv: doc: Add relative doc to describe RISC-V

Add documents to describe NX25 and AE250.
Also update other documents for RISC-V.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>
diff --git a/doc/README.standalone b/doc/README.standalone
index 659a12f..28ebde1 100644
--- a/doc/README.standalone
+++ b/doc/README.standalone
@@ -58,6 +58,7 @@
 	Blackfin	0x00001000	0x00001000
 	NDS32		0x00300000	0x00300000
 	Nios II		0x02000000	0x02000000
+	RISC-V		0x00600000	0x00600000
 
    For example, the "hello world" application may be loaded and
    executed on a PowerPC board with the following commands: