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: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>