target/riscv: Add the ePMP feature

The spec is avaliable at:
https://docs.google.com/document/d/1Mh_aiHYxemL0umN3GTTw8vsbmzHZ_nxZXgjgOUzbvc8

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 28c8855c80b0388a08c3ae009f5467e2b3960ce0.1618812899.git.alistair.francis@wdc.com

target/riscv/cpu.h

@@ -81,6 +81,7 @@
 enum {
     RISCV_FEATURE_MMU,
     RISCV_FEATURE_PMP,
+    RISCV_FEATURE_EPMP,
     RISCV_FEATURE_MISA
 };