[ARM] pxa/csb701: do not register devices on non-csb726 boads

csb701 driver can currently only be used on csb726 boards,
limit the csb701 devices registration to csb726 board.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Eric Miao <eric.miao@marvell.com>