arm64: Move /proc/cpuinfo handling code

This patch moves the /proc/cpuinfo handling code:

arch/arm64/kernel/{setup.c to cpuinfo.c}

No functional changes

Signed-off-by: Suzuki K. Poulose <suzuki.poulose@arm.com>
Tested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>