ppc4xx: t3corp: Add support for the Xilinx DS617 flash chip

The t3corp board has an Xilinx DS617 flash chip connected to the
onboard FPGA. This patch adds support for these chips. Board
specific flash accessor functions are needed, since the chips
can only be read correctly in 16bit mode.

Additionally the FPGA chip-selects are configured for device-paced
transfers (ready is enabled).

Signed-off-by: Stefan Roese <sr@denx.de>