spi_mpc83xx: add small delay after asserting chip-select line

This is needed for some underlaying GPIO controllers that may be a bit
slow, or if chip-select signal need some time to stabilize.

For what it's worth, we already have the similar delay for chip-select
de-assertion case.

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>