serial: ns16550: Try get serial clock rate from DT before CLK

Initializing a clock driver to read a known static clock rate can take
some time at U-Boot proper pre-reloc phase.

Change to first try and read clock rate from DT to speed up boot time,
fall back to getting the clock rate from clock driver.

This help reduce boot time by around:
- ~35ms on a Radxa ROCK Pi 4 (RK3399)
- ~15ms on a Radxa ZERO 3W (RK3566)
Time that is wasted getting a static rate known at compile time.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>