board/aarch64: disable BootROM debug on RPi's

Now that we've got everything working, it's just annoying, so disable it
but leave it in with a comment.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/aarch64/raspberrypi-rpi64/config.txt

@@ -1,3 +1,9 @@
+# Raspberry Pi bootloader (start.elf) configuration
+#
+# uart_2ndstage=1 - Enable early boot debug output from the bootloader
+#                   Useful for debugging boot failures before U-Boot starts
+#                   Comment out for production to reduce boot noise
+
 [all]
 arm_64bit=1
 
@@ -12,7 +18,7 @@ dtoverlay=infix-key
 disable_overscan=1
 
 enable_uart=1
-uart_2ndstage=1
+#uart_2ndstage=1
 
 [pi3]
 start_file=start.elf