openrisc: Add support for liteuart emergency printing

This patch adds support for sending emergency print output, such as
unhandled exception details, to a liteuart serial device.  This is the
default device available on litex platforms.

If a developer want to use this they should update UART_BASE_ADD
to the address of liteuart.

Signed-off-by: Stafford Horne <shorne@gmail.com>

Author: stffrdhrn

arch/openrisc/kernel/head.S

@@ -1555,7 +1555,18 @@ ENTRY(_emergency_putc)
 	l.movhi r4,hi(UART_BASE_ADD)
 	l.ori	r4,r4,lo(UART_BASE_ADD)
 
-#if defined(CONFIG_SERIAL_8250)
+#if defined(CONFIG_SERIAL_LITEUART)
+	/* Check OFF_TXFULL status */
+1:      l.lwz	r5,4(r4)
+	l.andi	r5,r5,0xff
+	l.sfnei	r5,0
+	l.bf	1b
+	 l.nop
+
+	/* Write character */
+	l.andi	r7,r7,0xff
+	l.sw	0(r4),r7
+#elif defined(CONFIG_SERIAL_8250)
 	/* Check UART LSR THRE (hold) bit */
 	l.addi  r6,r0,0x20
 1:      l.lbz   r5,5(r4)