sandbox: Introduce dummy remoteproc nodes

Introduce dummy devices for sandbox remoteproc device and enable it by
default

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 15e7b50..67ae99b 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -63,3 +63,5 @@
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
 CONFIG_UT_ENV=y
+CONFIG_REMOTEPROC_SANDBOX=y
+CONFIG_CMD_REMOTEPROC=y