RISC-V: Increase range and default value of NR_CPUS

Currently, the range and default value of NR_CPUS is too restrictive
for high-end RISC-V systems with large number of HARTs. The latest
QEMU virt machine supports upto 512 CPUs so the current NR_CPUS is
restrictive for QEMU as well. Other major architectures (such as
ARM64, x86_64, MIPS, etc) have a much higher range and default
value of NR_CPUS.

This patch increases NR_CPUS range to 2-512 and default value to
XLEN (i.e. 32 for RV32 and 64 for RV64).

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Link: https://lore.kernel.org/r/20220420112408.155561-1-apatel@ventanamicro.com/
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>