[MIPS] DECstation: Optimised early printk()

This is an optimised implementation of early printk() for the DECstation.  
After the recent conversion to a MIPS-specific generic routine using a 
character-by-character output the performance dropped significantly.  
This change reverts to the previous speed -- even at 9600 bps of the 
serial console the difference is visible with a naked eye; I presume for a 
framebuffer it is even worse (it may depend on exactly which one is used 
though).

 Additionally the change includes a fix for a problem that the old 
implementation had -- the format used would not actually limit the length 
of the string output.  This new implementation uses a local buffer to deal 
with it -- even with this additional copying it is much faster than the 
generic function.

 Plus this driver is registered much earlier than the generic one, 
allowing one to see critical messages, such as one about an incorrect CPU 
setting used, that are produced beforehand. :-)

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/dec/prom/init.c

@@ -103,6 +103,9 @@ void __init prom_init(void)
 	if (prom_is_rex(magic))
 		rex_clear_cache();
 
+	/* Register the early console.  */
+	register_prom_console();
+
 	/* Were we compiled with the right CPU option? */
 #if defined(CONFIG_CPU_R3000)
 	if ((current_cpu_data.cputype == CPU_R4000SC) ||