target/riscv/pmp.c: pmpcfg_csr_read returns bogus value on RV64

target/riscv/pmp.c: pmpcfg_csr_read returns bogus value on RV64

pmp_read_cfg() returns 8-bit value, which is combined together to form a single pmpcfg CSR.
The default promotion rules will result in an integer here ("i*8" is integer, which
flows through) resulting in a 32-bit signed value on most hosts.
That's bogus on RV64I, with the high bits of the CSR being wrong.

Signed-off-by: Dayeol Lee <dayeol@berkeley.edu>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>