imx6: Fix serial baud calculation for nitrogen6sx

The nitrogen6sx platform has a different input clock frequency to the
uart device compared to the sabre platform. This commit corrects the
constant definition and adds a small comment explaining how the constant
is not quite the value of the input uart clock frequency.

Signed-off-by: Kent McLeod <kent@kry10.com>

libplatsupport/plat_include/imx6/platsupport/plat/serial.h

diff --git a/libplatsupport/plat_include/imx6/platsupport/plat/serial.h b/libplatsupport/plat_include/imx6/platsupport/plat/serial.h
index 431e4aa..da53d8a 100644
--- a/libplatsupport/plat_include/imx6/platsupport/plat/serial.h
+++ b/libplatsupport/plat_include/imx6/platsupport/plat/serial.h
@@ -23,7 +23,17 @@
 #define UART4_IRQ    61
 #define UART5_IRQ    62
 
+#ifdef CONFIG_PLAT_NITROGEN6SX
+// seL4 starts on the Nitrogen6_SoloX platform with the UART input
+// clock frequency set to 24MHz. This constant is used in baud rate
+// calculations where the input frequency has already been divided by 2.
+#define UART_REF_CLK 12000000
+#else
+// This constant assumes that the UART input clock frequency is 80179200Hz
+// and gets divided by 2 before performing baud calculations where this
+// constant is used.
 #define UART_REF_CLK 40089600
+#endif
 
 
 #if defined(CONFIG_PLAT_SABRE)