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>

include/configs/rpi_b.h

@@ -23,6 +23,7 @@
 #define CONFIG_ARM1176
 #define CONFIG_BCM2835
 #define CONFIG_ARCH_CPU_INIT
+#define CONFIG_SYS_DCACHE_OFF
 /*
  * 2835 is a SKU in a series for which the 2708 is the first or primary SoC,
  * so 2708 has historically been used rather than a dedicated 2835 ID.