arm: mvebu: armada-3720: remove unused config option

The config option CONFIG_DEBUG_UART_CLOCK is not used by Armada 3720's
serial driver (it wasn't even before the recent update of that driver).

Even if it was used, the value was incorrect (the frequency of the clock
is 25 MHz, not 25.8048 MHz).

Remove it from config files and set the default value to 0.

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