[MTD] [NAND] Support multiple chips in CAFÉ driver

The CAFÉ can handle two chip on separate chipselect lines. Hook up the
undocumented chipselect bits in the driver and probe both.

In the case of OLPC, it's not actually two separate devices -- it's a
single '1GiB' package with two 512MiB dies internally. So clear the
NAND_BBT_PERCHIP flag to treat it as a single chip for BBT purposes, and
make life easier for the firmware.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>