serial: a37xx: Use TBG as parent clock

Using TBG clock as parent clock for UART allows us using higher
baudrates than 230400.

Turris MOX with external FT232RL USB-UART works fine up to 3 MBaud
(which is maximum for this USB-UART controller), while EspressoBIN with
integrated pl2303 USB-UART also works fine up to 6 MBaud.

Slower baudrates with TBG as a parent clock can be achieved by
increasing TBG dividers and oversampling divider. When using the slowest
TBG clock, minimal working baudrate is 300.

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