ARM: rpi: rename rpi_b to rpi

The U-Boot port runs on a variety of RPi models, not just the B. So,
rename the port to something slightly more generic.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/rpi_defconfig b/configs/rpi_defconfig
new file mode 100644
index 0000000..9379cf0
--- /dev/null
+++ b/configs/rpi_defconfig
@@ -0,0 +1,2 @@
+CONFIG_ARM=y
+CONFIG_TARGET_RPI=y