MIPS: Eleminate filenames from comments

They tend to get not updated when files are moved around or copied and
lack any obvious use.  While at it zap some only too obvious comments and
as per Shinya's suggestion, add a copyright header to extable.c.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>