docs(a3k): update documentation about DEBUG mode for UART

DEBUG mode can be enabled without any issue for Armada 37xx and also for
other A7K/A8K/CN913x. There is no incompatibility with Xmodem protocol
like it was written before, because Armada 37xx UART images do not print
anything on UART during image transfer and A7K/A8K/CN913x BLE image
automatically turn off debugging output when booting over UART. Looks
like this incorrect information is some relict from the past.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I85adc3c21036656b4620c4692e04330cad11ea2f