target/riscv: Fix the translation of physical address

The real physical address should add the 12 bits page offset. It also
causes the PMP wrong checking due to the minimum granularity of PMP is
4 byte, but we always get the physical address which is 4KB alignment,
that means, we always use the start address of the page to check PMP for
all addresses which in the same page.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <370a983d0f9e8a9a927b9bb8af5e7bc84b1bf9b1.1595924470.git.zong.li@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>