Blackfin: bf537-stamp: tweak i2c address for ad5280 add-on tftlcd board

The predefined i2c address 0x2c doesn't match the configuration of the
ad5280 PINs AD0 and AD1 on the tftlcd add-on board.  Both AD0 and AD1
are of voltage 3.3V, which means the i2c address should be 0x2F.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>