arm: mvebu: Enable BootROM output on A38x

BootROMs on pre-A38x SoCs enabled its output on UART by default, but A38x'
BootROM has its output on UART disabled by default.

To enable BootROM output on A38x SoC, it is required to set DEBUG flag
(which only enables BootROM output and nothing more) in kwbimage. For UART
images this DEBUG flag is ignored by BootROM.

Enable kwbimage DEBUG flag for all A38x boards.

With this change BootROM prints the following (success) information on UART
before booting U-Boot kwbimage:

  BootROM - 1.73
  Booting from SPI flash

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>