vexpress64: specify CONFIG_SYS_FLASH_CFI_WIDTH

vexpress64: specify CONFIG_SYS_FLASH_CFI_WIDTH

Testing on the ARM Versatile Express Juno board showed intermittent CFI
flash detection problems.

Debug output in exception cases showed that the NOR flash was returning
incorrect values to a query command:

fwc addr 0000000008000000 cmd f0 f0f0f0f0 32bit x 8 bit
fwc addr 0000000008000000 cmd ff ffffffff 32bit x 8 bit
fwc addr 0000000008000154 cmd 98 98989898 32bit x 8 bit
is= cmd 51(Q) addr 0000000008000040 is= 00800051 51515151
fwc addr 0000000008001554 cmd 98 98989898 32bit x 8 bit
is= cmd 51(Q) addr 0000000008000040 is= 00800051 51515151
fwc addr 0000000008000000 cmd f0 00f000f0 32bit x 16 bit
fwc addr 0000000008000000 cmd ff 00ff00ff 32bit x 16 bit
fwc addr 0000000008000154 cmd 98 00980098 32bit x 16 bit
is= cmd 51(Q) addr 0000000008000040 is= 00800051 00510051
fwc addr 0000000008001554 cmd 98 00980098 32bit x 16 bit
is= cmd 51(Q) addr 0000000008000040 is= 00800051 00510051

Debugging showed that the problem could be solved by preventing u-boot
from scanning different flash widths by specifying
CONFIG_SYS_FLASH_CFI_WIDTH so that only using the 'correct' width was
used.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>