ARM: vexpress_ca9x4: Add missing flash width config option

Allow for a proper configuration of CFI flash banks avaialble on the vexpress_ca9x4
board. Without this option, the CFI flash incorrectly detects that the board has two
banks of 32MB flash devices, while in reality, the board provides
two flash banks, each with 64MB size. As a result, it becomes impossible to e.g. to
save u-boot env in flash. According to device tree for this board and
its implementation in QEMU, the CFI width should be set to 32 bits.

After applying this fix, CFI flash will correctly detect both flash
banks each with a size of 64MB. As as result the functionality of e.g. saving u-boot
env will work correctly.

Tested on QEMU 6.2.0.

Cc: Kristian Amlie <kristian.amlie@northern.tech>
Signed-off-by: Patryk Biel <pbiel7@gmail.com>
Reviewed-by: Kristian Amlie <kristian.amlie@northern.tech>