arm64: Add support for Half precision floating point

ARMv8.2 extensions [1] include an optional feature, which supports
half precision(16bit) floating point/asimd data processing
instructions. This patch adds support for detecting and exposing
the same to the userspace via HWCAPs

[1] https://community.arm.com/groups/processors/blog/2016/01/05/armv8-a-architecture-evolution



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