arm64: defconfig: increase NR_CPUS default to 64

Raising the current maximum limit to 64. This is needed for Cavium's
Thunder systems that will have at least 48 cores per die.

The change keeps the current memory footprint in cpu mask structures.
It does not break existing code. Setting the maximum to 64 cpus still
boots systems with less cpus.

Mark's Juno happily booted with a NR_CPUS=64 kernel.

Tested on our Thunder system with 48 cores. We could see interrupts to
all cores.

Cc: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NRobert Richter <rrichter@cavium.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/Kconfig

@@ -253,11 +253,11 @@ config SCHED_SMT
 	  places. If unsure say N here.
 
 config NR_CPUS
-	int "Maximum number of CPUs (2-32)"
-	range 2 32
+	int "Maximum number of CPUs (2-64)"
+	range 2 64
 	depends on SMP
 	# These have to remain sorted largest to smallest
-	default "8"
+	default "64"
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs"