target/riscv/cpu.c: error out if EPMP is enabled without PMP

target/riscv/cpu.c: error out if EPMP is enabled without PMP

Instead of silently ignoring the EPMP setting if there is no PMP
available, error out informing the user that EPMP depends on PMP
support:

$ ./qemu-system-riscv64 -cpu rv64,pmp=false,x-epmp=true
qemu-system-riscv64: Invalid configuration: EPMP requires PMP support

This will force users to pick saner options in the QEMU command line.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230222185205.355361-6-dbarboza@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>