ARM: rpi_b: disable rpi_b dcache explicitly

There appears to be no implementation of flush_dcache_range() for
ARM1176, so explicitly disable dcache support to avoid references to
that function from the LCD core in the next patch. This was presumably
not noticed before simply because no drivers for the rpi_b were
attempting DMA.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>