arm64: kconfig: increase NR_CPUS range to 2-4096.

Raise the maximum CPU limit to 4096 in preparation for upcoming
platforms with large core counts.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ganapatrao Kulkarni <gkulkarni@caviumnetworks.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>