PmpPluginOld: fix NAPOT address calculation overflow issue #374
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Because
the start and end address registers need to be XLEN + 4 bit wide to avoid overflows.
If such an overflow occurs, it may be that the region does not cover any address, an issue uncovered in the Tock LiteX + VexRiscv CI during a PMP infrastructure redesign in the Tock OS 1.
This commit has been tested on Tock's redesigned PMP infrastructure, and by inspecting all of the intermediate signals in the PMP address calculation through a Verilator trace file. It works correctly for various NAPOT and TOR addresses, and I made sure that the edge cases of pmpaddrX = [0x00000000, 0x7FFFFFFF, 0xFFFFFFFF] are all handled.