libchip/ns16550: Allow user calculate baud divisor

This patch will allow the user to pass a function to calculate
the baud divisor.
This will allow for more flexibility, since for some BSPs
like raspberrypi, the calculation of baud divisor is different
from what is in the current driver.

bsps/shared/dev/serial/ns16550-context.c

@@ -112,6 +112,8 @@ static uint32_t NS16550_GetBaudDivisor(ns16550_context *ctx, uint32_t baud)
       NS16550_FRACTIONAL_DIVIDER,
       fractionalDivider
     );
+  } else if (ctx->calculate_baud_divisor != NULL) {
+    baudDivisor = ctx->calculate_baud_divisor(ctx, baud);
   }
 
   return baudDivisor;