powerpc/idle/6xx: Use r1 with CURRENT_THREAD_INFO()

Make sure CURRENT_THREAD_INFO() is used with r1 which is the virtual
address of the stack, in order to ease the switch to r2 when we enable
THREAD_INFO_IN_TASK, as we have no register having the phys address of
current.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Split out of larger patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>