[NET] smc91x: prepare SMC_USE_PXA_DMA to be specified in platform data

Now that the original SMC_USE_PXA_DMA specific code will always being
built if CONFIG_ARCH_PXA is defined, so to make this part of the code
to be PXA public, and still prevent it from being built if support of
PXA is not selected.

A SMC91X_USE_DMA flag is added to the platform data to allow platform
to choose its usage of DMA. Note this flag itself is so named to be
generic enough (assuming other platforms can also use DMA).

It keeps backward compatibility to set the SMC91X_USE_DMA flag if
SMC_USE_PXA_DMA is still defined.

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Acked-by: Nicolas Pitre <nico@cam.org>
Acked-by: Jeff Garzik <jgarzik@pobox.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>