marvell: uart: a3720: Implement console_a3700_core_flush

Implementation is simple, just wait for the TX FIFO to be empty.

Without this patch TF-A on A3720 truncate the last line:

  NOTICE:  BL31: Built : 16:1

With this patch TF-A on A3720 print correctly also the last line:

  NOTICE:  BL31: Built : 19:03:31, Dec 23 2020
Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I2f2ea42beab66ba132afdb400ca7898c5419db09

drivers/marvell/uart/a3700_console.S

@@ -232,6 +232,11 @@ endfunc console_a3700_getc
 	 * ---------------------------------------------
 	 */
 func console_a3700_core_flush
+	/* Wait for the TX FIFO to be empty */
+1:	ldr	w1, [x0, #UART_STATUS_REG]
+	and	w1, w1, #UARTLSR_TXFIFOEMPTY
+	cmp	w1, #UARTLSR_TXFIFOEMPTY
+	b.ne	1b
 	ret
 endfunc console_a3700_core_flush