ARM64: Make HZ configurable

ARM64: Make HZ configurable

The default value of 100HZ is not fast enough for a smooth
user-experience because preemption isn't slick enough.

Let platforms configure HZ via defconfig to suit their needs.

Bug 1695965

Change-Id: I38874e802cc5a796049b57d0622d0c9a9c9b438c
Signed-off-by: Sai Gurrappadi <sgurrappadi@nvidia.com>
Reviewed-on: http://git-master/r/818991
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Tested-by: Bharat Nihalani <bnihalani@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User