target-xtensa: xtfpga: fix FLASH interface width

FLASH chip on XTFPGA boards is connected with 16-bit-wide interface.
Latest U-Boot can see the difference and does not work correctly with
32-bit-wide interface.
Set FLASH chip 'width' property to 2.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>