spi_mpc83xx: rename spi_83xx.c to spi_8xxx.c

The driver handles MPC83xx, MPC85xx and MPC86xx SPI controllers, so rename
the file for clarity.

Suggested-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>