target/riscv: Add ePMP CSR access functions

target/riscv: Add ePMP CSR access functions

Signed-off-by: Hongzheng-Li <Ethan.Lee.QNL@gmail.com>
Signed-off-by: Hou Weiying <weiying_hou@outlook.com>
Signed-off-by: Myriad-Dreamin <camiyoru@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 270762cb2507fba6a9eeb99a774cf49f7da9cc32.1618812899.git.alistair.francis@wdc.com
[ Changes by AF:
 - Rebase on master
 - Fix build errors
 - Fix some style issues
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>