arm: mvebu: a37xx: Enable more baudrates

Extend CONFIG_SYS_BAUDRATE_TABLE and include all standard baudrates and
also nonstandard up to the 6 MBaud. U-Boot's A3720 UART driver can use
baudrates from 300 Baud to 6 MBaud.

This changes all A3720 boards, since all of them include either
mvebu_armada-37xx.h or turris_mox.h config file.

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