target/riscv: Change the TLB page size depends on PMP entries.

The minimum granularity of PMP is 4 bytes, it is small than 4KB page
size, therefore, the pmp checking would be ignored if its range doesn't
start from the alignment of one page. This patch detects the pmp entries
and sets the small page size to TLB if there is a PMP entry which cover
the page size.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <6b0bf48662ef26ab4c15381a08e78a74ebd7ca79.1595924470.git.zong.li@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>