[ARM] 3972/1: AT91: Update board.h

Replace the 'is_b' variable with 'slot_b' in at91_mmc_data.
Also add the new 'chipselect' variable for CF/PCMCIA and 'bus_width_16'
variable for NAND.

This (and previous patches) will unfortunately break the current MMC,
USB Gadget and PCMCIA drivers.  Updates and fixes for those drivers will
be submitted to the various subsystem maintainers.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>