x86: Allow use of serial soon after relocation

At present on x86 machines with use cache-as-RAM, the memory goes away just
before board_init_r() is called. This means that serial drivers are
no-longer unavailable, until initr_dm() it called, etc.

Any attempt to use printf() within this period will cause a hang.

To fix this, mark the serial devices as 'unavailable' when it is no-longer
available. Bring it back when serial_initialize() is called. This means that
the debug UART will be used instead for this period.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index f154eb1..0ce5c44 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -115,7 +115,7 @@
 /* Called after relocation */
 void serial_initialize(void)
 {
-	serial_find_console_or_panic();
+	serial_init();
 }
 
 static void _serial_putc(struct udevice *dev, char ch)