ARM64_CORE_REG() is currently only used to generate the KVM
register IDs for registers that happen to be 64 bits in size, so
KVM_REG_SIZE_U64 is hard-coded in the definition.

To enable this macro to generate correct encodings for the FPSIMD
registers too (which are a mix of 128-bit and 32-bit registers),
this patch extends the macro to encode the correct size for each
class of register in KVM_REG_ARM_CORE.

The approach is crude, but because the KVM_REG_ARM_CORE ID
arrangement is ABI, it's not expected to evolve.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>