openrisc: head: Move init strings to rodata section

The strings used during the head/init phase of openrisc bootup were
stored in the executable section of the binary.

This causes compilation to fail when using CONFIG_DEBUG_INFO with
error:
 Error: unaligned opcodes detected in executable segment
Signed-off-by: Stafford Horne <shorne@gmail.com>

arch/openrisc/kernel/head.S

@@ -1530,6 +1530,7 @@ ENTRY(_early_uart_init)
 	l.jr	r9
 	l.nop
 
+	.section .rodata
 _string_copying_linux:
 	.string "\n\n\n\n\n\rCopying Linux... \0"