ARM: ixp4xx: Make dma_set_coherent_mask common, correct implementation

Non-PCI devices can use the entire 32-bit range, PCI dittos are
limited to the first 64MiB.

Also actually setup coherent_dma_mask.

The patch has been verified on a board with 128MiB memory, one
ipx4xx_eth device and a e100 PCI device.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>