spi: add spi_ppc4xx driver

This adds a SPI driver for the SPI controller found in the IBM/AMCC
4xx PowerPC's.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Wolfgang Ocker <weo@reccoware.de>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Steven A. Falco <sfalco@harris.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>