rockchip: Correct UART reference clock value

The Rockchip UARTs are tied directly to the 24MHz oscillator and are
thus clocked with exactly 24MHz. The reasons why our code instead uses
some 23.xxMHz value have long been lost in time. For the current shared
8250 implementation, the baud rate divisor for 115200 would be the same.

Correcting this does make the information in the coreboot table more
accurate and help payloads chose a better divisor, though.

Change-Id: Ieceb07760178f8ddbb5936f8742b78f8def4072d
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/23556
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/rockchip/common/uart.c

@@ -14,11 +14,12 @@
  */
 
 #include <console/uart.h>
+#include <soc/clock.h>
 #include <stdint.h>
 
 unsigned int uart_platform_refclk(void)
 {
-	return 23040000;
+	return OSC_HZ;
 }
 
 uintptr_t uart_platform_base(int idx)