sonata: now using a macro to set default baud rate

This will allow the default baud rate to be set from the board
configuration.

sdk/boards/sonata-0.2.json

diff --git a/sdk/boards/sonata-0.2.json b/sdk/boards/sonata-0.2.json
index 6faa442..308904a 100644
--- a/sdk/boards/sonata-0.2.json
+++ b/sdk/boards/sonata-0.2.json
@@ -71,6 +71,7 @@
         "SUNBURST",
         "SUNBURST_SHADOW_BASE=0x30000000",
         "SUNBURST_SHADOW_SIZE=0x4000",
+        "DEFAULT_UART_BAUD_RATE=115200",
         "ipconfigDRIVER_INCLUDED_RX_IP_CHECKSUM=1",
         "ipconfigDRIVER_INCLUDED_TX_IP_CHECKSUM=1"
     ],

sdk/include/platform/sunburst/platform-uart.hh

diff --git a/sdk/include/platform/sunburst/platform-uart.hh b/sdk/include/platform/sunburst/platform-uart.hh
index 8ac457b..09f217d 100644
--- a/sdk/include/platform/sunburst/platform-uart.hh
+++ b/sdk/include/platform/sunburst/platform-uart.hh
@@ -4,6 +4,10 @@
 #include_next <platform-uart.hh>
 #pragma pop_macro("CHERIOT_PLATFORM_CUSTOM_UART")
 
+#ifndef DEFAULT_UART_BAUD_RATE
+#	define DEFAULT_UART_BAUD_RATE 921'600
+#endif
+
 /**
  * OpenTitan UART
  *
@@ -236,7 +240,7 @@
 		interruptEnable = interruptEnable & ~interrupt;
 	}
 
-	void init(unsigned baudRate = 115'200) volatile
+	void init(unsigned baudRate = DEFAULT_UART_BAUD_RATE) volatile
 	{
 		// Nco = 2^20 * baud rate / cpu frequency
 		const uint32_t Nco =