KVM: arm64: selftests: Remove ARM64_FEATURE_FIELD_BITS and its last user

KVM: arm64: selftests: Remove ARM64_FEATURE_FIELD_BITS and its last user

ARM64_FEATURE_FIELD_BITS is set to 4 but not all ID register fields are 4
bits. See for instance ID_AA64SMFR0_EL1. The last user of this define,
ARM64_FEATURE_FIELD_BITS, is the set_id_regs selftest. Its logic assumes
the fields aren't a single bits; assert that's the case and stop using the
define. As there are no more users, ARM64_FEATURE_FIELD_BITS is removed
from the arm64 tools sysreg.h header. A separate commit removes this from
the kernel version of the header.

Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>