spmi: Add sandbox test driver

This patch adds emulated spmi bus controller with part of
pm8916 pmic on it to sandbox and tests validating SPMI uclass.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/spmi/Makefile b/drivers/spmi/Makefile
index 99092eb..4ca65a9 100644
--- a/drivers/spmi/Makefile
+++ b/drivers/spmi/Makefile
@@ -5,3 +5,4 @@
 #
 
 obj-$(CONFIG_SPMI)  += spmi-uclass.o
+obj-$(CONFIG_SPMI_SANDBOX) += spmi-sandbox.o