target/riscv: Make PMP region count configurable

target/riscv: Make PMP region count configurable

Previously, the number of PMP regions was hardcoded to 16 in QEMU.
This patch replaces the fixed value with a new `pmp_regions` field,
allowing platforms to configure the number of PMP regions.

If no specific value is provided, the default number of PMP regions
remains 16 to preserve the existing behavior.

A new CPU parameter num-pmp-regions has been introduced to the QEMU
command line. For example:

	-cpu rv64, g=true, c=true, pmp=true, num-pmp-regions=8

Signed-off-by: Jay Chang <jay.chang@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250606072525.17313-3-jay.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>