microblaze: Add support for early console on mdm

Support mdm early console:
 - extend time for retries
 - add mdm compatible property

Signed-off-by: Michal Simek <monstr@monstr.eu>
diff --git a/arch/microblaze/kernel/early_printk.c b/arch/microblaze/kernel/early_printk.c
index e21390d..d26d92d 100644
--- a/arch/microblaze/kernel/early_printk.c
+++ b/arch/microblaze/kernel/early_printk.c
@@ -35,7 +35,7 @@
 	 * we'll never timeout on a working UART.
 	 */
 
-	unsigned retries = 10000;
+	unsigned retries = 1000000;
 	/* read status bit - 0x8 offset */
 	while (--retries && (in_be32(base_addr + 8) & (1 << 3)))
 		;