tty/serial: 8250_early: Add support for PXA UARTs

The PXA variant of the 8250 UART adds a UART enable bit which must not
be cleared. Make the earlycon support maintain this bit if it is set.
This implies some initialization of the UART, but we cannot
unconditionally set the bit as some other variants require this bit to
be clear for other functions.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_early.c

@@ -93,10 +93,10 @@ static void __init early_serial8250_write(struct console *console,
 	struct uart_port *port = &early_device->port;
 	unsigned int ier;
 
-	/* Save the IER and disable interrupts */
+	/* Save the IER and disable interrupts preserving the UUE bit */
 	ier = serial8250_early_in(port, UART_IER);
 	if (ier)
-		serial8250_early_out(port, UART_IER, 0);
+		serial8250_early_out(port, UART_IER, ier & UART_IER_UUE);
 
 	uart_console_write(port, s, count, serial_putc);
 
@@ -127,9 +127,11 @@ static void __init init_port(struct earlycon_device *device)
 	struct uart_port *port = &device->port;
 	unsigned int divisor;
 	unsigned char c;
+	unsigned int ier;
 
 	serial8250_early_out(port, UART_LCR, 0x3);	/* 8n1 */
-	serial8250_early_out(port, UART_IER, 0);	/* no interrupt */
+	ier = serial8250_early_in(port, UART_IER);
+	serial8250_early_out(port, UART_IER, ier & UART_IER_UUE); /* no interrupt */
 	serial8250_early_out(port, UART_FCR, 0);	/* no fifo */
 	serial8250_early_out(port, UART_MCR, 0x3);	/* DTR + RTS */