[MIPS] DECstation: Optimised early printk()

[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>