target/riscv/pmp.c: Fix pmp_decode_napot()

target/riscv/pmp.c: Fix pmp_decode_napot()

Currently, start and end address of a PMP region are not decoded
correctly by pmp_decode_napot().

Let's say we have a 128KB PMP region with base address as 0x80000000.
Now, the PMPADDRx CSR value for this region will be 0x20003fff.

The current pmp_decode_napot() implementation will decode PMPADDRx
CSR as t1=14, base=0x100000000, and range=0x1ffff whereas it should
have decoded PMPADDRx CSR as t1=14, base=0x80000000, and range=0x1fff.

This patch fixes the base value decoding in pmp_decode_napot() when
PMPADDRx CSR is not -1 (i.e. 0xffffffffffffffff).

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>