KVM: Allow 2048-bit register access via ioctl interface

The Arm SVE architecture defines registers that are up to 2048 bits
in size (with some possibility of further future expansion).

In order to avoid the need for an excessively large number of
ioctls when saving and restoring a vcpu's registers, this patch
adds a #define to make support for individual 2048-bit registers
through the KVM_{GET,SET}_ONE_REG ioctl interface official.  This
will allow each SVE register to be accessed in a single call.

There are sufficient spare bits in the register id size field for
this change, so there is no ABI impact, providing that
KVM_GET_REG_LIST does not enumerate any 2048-bit register unless
userspace explicitly opts in to the relevant architecture-specific
features.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>