Merge pull request riscv#1807 from trdthg/add-seed-csr

Add seed CSR to CSR table

Author: aswaterman

src/priv-csrs.adoc

@@ -190,6 +190,12 @@ Vector register length in bytes.
 |`ssp` +
 |Shadow Stack Pointer. +
 
+4+^|Unprivileged Entropy Source Extension CSR
+|`0x015` +
+|URW +
+|`seed` +
+|Seed for cryptographic random bit generators. +
+
 4+^|Unprivileged Counter/Timers
 
 |`0xC00` +