next-cube.c: move static phase variable to NextRtc

next-cube.c: move static phase variable to NextRtc

The phase variable represents part of the state machine used to clock data out
of the NextRtc device.

Note that this is a migration break for the NeXTRtc struct, but as nothing will
currently boot then we simply bump the migration version for now.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20231220131641.592826-8-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>