lib: utils/serial: support 'reg-offset' property

reg-offset property is used for offset to apply to the mapbase
from the start of the registers in 8250 UART. In Linux kernel,
it has been handled in 8250 UART driver.

dt-bindings:
<linux>/Documentation/devicetree/bindings/serial/8250.yaml

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

platform/template/platform.c

@@ -79,7 +79,7 @@ static int platform_console_init(void)
 {
 	/* Example if the generic UART8250 driver is used */
 	return uart8250_init(PLATFORM_UART_ADDR, PLATFORM_UART_INPUT_FREQ,
-			     PLATFORM_UART_BAUDRATE, 0, 1);
+			     PLATFORM_UART_BAUDRATE, 0, 1, 0);
 }
 
 /*