xen/arm64: head: Don't clobber x30/lr in the macro PRINT

The current implementation of the macro PRINT will clobber x30/lr. This
means the user should save lr if it cares about it.

Follow-up patches will introduce more use of PRINT in place where lr
should be preserved. Rather than requiring all the users to preserve
lr, the macro PRINT is modified to save and restore it.

While the comment state x3 will be clobbered, this is not the case. So
PRINT will use x3 to preserve lr.

Lastly, take the opportunity to move the comment on top of PRINT and use
PRINT in init_uart. Both changes will be helpful in a follow-up patch.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/arm64/head.S

@@ -78,12 +78,17 @@
  *  x30 - lr
  */
 
-/* Macro to print a string to the UART, if there is one.
- * Clobbers x0-x3. */
 #ifdef CONFIG_EARLY_PRINTK
-#define PRINT(_s)           \
-        adr   x0, 98f ;     \
-        bl    puts    ;     \
+/*
+ * Macro to print a string to the UART, if there is one.
+ *
+ * Clobbers x0 - x3
+ */
+#define PRINT(_s)          \
+        mov   x3, lr ;     \
+        adr   x0, 98f ;    \
+        bl    puts    ;    \
+        mov   lr, x3 ;     \
         RODATA_STR(98, _s)
 #else /* CONFIG_EARLY_PRINTK */
 #define PRINT(s)
@@ -622,9 +627,8 @@ init_uart:
 #ifdef EARLY_PRINTK_INIT_UART
         early_uart_init x23, 0
 #endif
-        adr   x0, 1f
-        b     puts
-RODATA_STR(1, "- UART enabled -\r\n")
+        PRINT("- UART enabled -\r\n")
+        ret
 
 /* Print early debug messages.
  * x0: Nul-terminated string to print.