xen: arm: reorder registers in struct cpu_user_regs.

xen: arm: reorder registers in struct cpu_user_regs.

Primarily this is so that they are ordered in the same way as the
mapping from arm64 x0..x31 registers to the arm32 registers, which is
just less confusing for everyone going forward.

It also makes the implementation of select_user_regs in the next patch
slightly simpler.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>