tools: kwboot: Increase delay after changing baudrate in ARM code

Increase loop cycles from 600000 to 2998272, which should increase delay
from 1ms to about 5ms on 1200 MHz CPU.

The Number 2998272 was chosen as the nearest value around 3000000 which can
be encoded into one ARM mov instruction. It avoids usage of movt instruction
which is not supported by ARMv5te cores.

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