[ARM] pxa: add support for CSB701 baseboard

CSB701 is one of baseboards that can be used with CSB726 SOM.
This currently adds support for button and LED on the board.
More to come later.

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