dm: test: Add a test for the ram uclass

Add a test to confirm that we can probe this device and get information on
the available RAM.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 907554f..99dc00f 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -47,3 +47,4 @@
 CONFIG_UT_ENV=y
 CONFIG_CLK=y
 CONFIG_RESET=y
+CONFIG_RAM=y