ARM: KVM: Rename struct coproc_reg::is_64 to is_64bit

As we're going to play some tricks on the struct coproc_reg,
make sure its 64bit indicator field matches that of coproc_params.

Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>