arm: mvebu: a38x: Define supported UART baudrates

arm: mvebu: a38x: Define supported UART baudrates

Define all standard baudrates plus 3 non-standard high speed:
3125000 4000000 5150000

3125000 matches divisor 5 with 250 MHz TCLK and divisor 4 with 200 MHz TCLK.
4000000 is the rounded value for divisor 4 with 250 MHz TCLK (3906250) and
divisor 3 with 200 MHz TCLK (4166666).

5150000 is the rounded value (5208333) for divisor 3 with 250 MHz TCLK.
Testing showed that rounded value is more stable then exactly calculated.
And it is the highest possible baudrate which is stable on A38x platform.

Any other baudrate values above 2500000 are unstable, which is reason why
e.g. standard value 3000000 is not defined, and it is needed to use
non-standard value 3125000.

Tested all defined UART baudrates on Turris Omnia (A38x with 250 MHz TCLK).

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