target/riscv/pmp: fix NAPOT range computation overflow

target/riscv/pmp: fix NAPOT range computation overflow

There is an overflow with the current code where a pmpaddr value of
0x1fffffff is decoded as sa=0 and ea=0 whereas it should be sa=0 and
ea=0xffffffff.

Fix that by simplifying the computation. There is in fact no need for
ctz64() nor special case for -1 to achieve proper results.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <rq81o86n-17ps-92no-p65o-79o88476266@syhkavp.arg>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>