arm: rpi: Switch to standard boot

Drop use of the distro scripts and use standard boot instead.

We don't need to specify the mmc devices individually, since they are
used in order from 0 to 2, and standard boot uses that order anyway.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/rpi_3_32b_defconfig b/configs/rpi_3_32b_defconfig
index 80885aa..305f1da 100644
--- a/configs/rpi_3_32b_defconfig
+++ b/configs/rpi_3_32b_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_SYS_LOAD_ADDR=0x1000000
+CONFIG_BOOTSTD_DEFAULTS=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_USE_PREBOOT=y